The original Tiger tank had a very large amount of problems, the greatest of which was that Allied medium tanks had equivalent firepower in 1944, just over a year after the Tiger first saw combat, but at half the weight. The Germans realized this, and the Tiger B/Tiger II/King Tiger, the development of which began before the first Tiger left the factory, had very little in common with the original Tiger tank. However, even the new Tiger tank was vulnerable to the gun of the Soviet IS-2 and American Pershing at its inception, in addition to lighter guns (17 pdr and D-10) mounted on medium tanks and medium tank destroyers, which again caused the Germans to seek a replacement design. The heavy Maus and E-100 tanks that would be impervious (at least frontally) to these guns ended up being cancelled because they were simply impractical. The decreasing quality of German armour meant that armour had to grow thicker and thicker in order to resist Allied weapons, making these superheavy tanks incredibly impractical, even compared to the Tigers.

By the time the German army was allowed to rebuild after the war, the heavy tank concept has taken enormous steps forward. The Soviets built the IS-7, a heavy tank with the size and weight of the King Tiger and enough armour protection to be impervious to the 128 mm gun of the Jagdtiger and Maus. That wasn't all, the new generation of heavy tanks, the Object 752 and Object 777 had as much effective armour as the E-100 while weighing a third as much. The Tigers, already not exactly progressive designs in the 1940s, looked hilariously primitive by comparison.

Sources:

Interrogation of Herr Stiele Von Heydekampf: German Tank & Engine Program

Ministry of Supply Armour Branch Report on Armour Quality & Vulnerability of Royal Tiger

Otchet po ispytnaniyu snaryadnym obstrelom lobovykh detaley korpusa i bashni nemetskogo tyazhelogo tanka Tigr B

Yuri Pasholok, Panzerkampfwagen Maus

Yuri Pasholok, Neschastlivye Tri Semyorki

Nikolai Nevsky, IS-7 Titan Opozdavshiy na Voynu

While there were a lot of problems and drawbacks with the German heavy tanks, the real reason is that armored doctrine simply shifted away from the 'Heavy' tank as an important weapon and towards an all around Main Battle Tank concept.

To get to the Leopard, weve first got to talk a little bit about how West Germany rearmed following World War Two. This process, for reasons I wont get into here, was very long and drawn out. Negotiations for rearmament didnt really even get started until 1950, when the Korean invasion shocked the world. Even still, foot dragging in Europe prevented much movement on the issue until 1954 when Germany was finally permitted to rebuild her military. At that point, the Germans had to create a completely new organization (the Wehrmacht had been completely destroyed after World War Two) and equip it with weapons which didnt yet exist. In the early 1950s this was done primarily through military assistance from the United States. This meant that for much of the 1950s and early 1960s, the German army used second hand American tanks.

But German designers were looking to build a domestic tank to replace imported materials. And, as you might expect, they wanted to produce cutting edge weapons. The late '50s had seen a pretty big shift in armored theory, especially in the United States. This was the result of decades worth of thinking, but essentially the US Army had slowly pushed away from an army built up of multiple tank models towards one which used a single design. Rather than have a heavy breakthrough tank (a project which the US never seemed to really get off the ground), a medium exploitation tank, and a light reconnaissance tank, the US began to pair its forces down to one tank. The Patton series of tanks really represented this switch to the MBT role. These tanks had guns which could kill most of the tanks in the Soviet armory, had armor which afforded them some protection, and yet maintained enough battlefield agility to relocate quickly.

Now, the Americans did fiddle with a heavy tank and a light tank (the M103 and the M551), but they generally found these tanks unsatisfactory on the battlefield, and quickly relegated them to niche specialist roles. Likewise, the British experimented with also experimented with heavier tank designs as well. But generally these were rejected in favor of lighter MBTs. The Germans realized why when they began to study the problem of the nuclear battlefield. NATO war planners had spent much of the 1960s, with and without German help, trying to figure out how to fight a hypothetical Pact invasion. And the answer they came up with involved a large number of nuclear weapons. Unlike World War Two, were large masses of men concentrated their effort into a narrow section of static enemy defense, the nuclear battlefield was fluid. Forces had to move fast and hit hard, and they especially had to avoid elaborate defensive works which were prime targets for nuclear attack. Heavy tanks simply lacked the speed and mobility to move rapidly across the battlefield and strike the enemy where he was most vulnerable. And this was a problem that wasnt just limited to the West. At nearly the same time the Soviets recognized that their heavy tank designs were simply too large and too good a target to be practical on the battlefield.

So, I would argue that the Leopard 1 was the German Army's first attempt to solve this question. They attempted to design a weapon which could operate on the nuclear battlefield utilizing a mixture of both off and domestic weapons. While their solution was uniquely German, it recognized the realities of the nuclear battlefield and it conformed to many of the standards set for by their allies. And, to the Tiger Series, unfortunately for such a beautiful tank, time had simply passed the heavy tank by. Had Germany rearmed in 1946, you may have seen a Tiger 3 tank. But by the mid 1960s, when the Leopard entered production, Heavy tanks were simply on the way out.

A note on Sources

This is compiled from a reading of several secondary sources some of which are out of print. The best general summary of tank development at this time would be Richard Ogorkiewicz's books, especially the Technology of Tanks. Another good book is The Cold War U.S. Army: Building Deterrence for Limited War. Now this book obviously focuses on the US Army, not the German, but it explores many of the problems associated with planning for the nuclear battlefield. It Also talks about the Army's institutional response to those challenges, from strategic and organizational changes, all the way down to weapons development and planning. It also does discuss the Germans extensively as they dealt with the problems of rearmament, and then integration into the overall European command.

While the other posters have discussed the relative technical demerits of the wartime "cats", it is important to realize that the Bundeswehr procurement procurement process was a highly politicized one. Under the auspices of Amt Blank, Adenauer erected a shadow defense ministry in 1950 under the leadership of Theodor Blank that sought to prepare the way for a West German rearmament. Adenauer saw that rearmament was desirable both as a guarantor of FRG security (especially if the US withdrew into isolation), and as a symbol that the FRG had regained its sovereignty. much of the proto-defense ministry favored a relatively large, sophisticated force that would guarantee FRG security. But Amt Blank's plans had to tread carefully because the issue of German rearmament was such a charged topic coming five years after the war. Much of Western European public opinion was against rearmament, as was a sizable percentage of the FRG's population, and most Western European policymakers favored a very limited form of rearmament. The British, for example, hemmed and hawed between supporting a limited FRG rearmament and a broader neutralization of both Germanys.

Therefore the Adenauer government had to thread between two contentious needles of both pushing for a quick rearmament and doing so in a way that did not alienate their would-be allies. Rearmament needed to be quick not only because the outbreak of the Korean War stimulated worst-case scenario thinking, but also a fait accompli would prevent any of the FRG's partners from getting cold feet, especially since it was known that the USSR would vigorously protest German rearmament. One strategy to accomplish this was through the European Defense Community (EDC), an attempt to create a common European army. The EDC proved to be a dead end though due to inter-European squabbles, but also because of disagreements over German force levels. Both Paris and London pushed for a much smaller German contribution than what Bonn wanted. Although the EDC proved to be a dead end, it did allow for the FRG to test the waters for rearmament and it found that while many Western European leaders were uncomfortable with the idea, they could stomach it. The result was that the FRG instead formed its own national army, the Bundeswehr, and incorporated it into an organization that had fallen into a degree of disuse during the protracted abortion of the EDC, NATO.

Bundeswehr procurement followed a two-pronged strategy under the newly-formed Ministry of Defense under Blank and his successor Franz Josef Strauß. The first part of the stratagem was to gain a maximum, modern force quickly. Termed "Broad Armament", this meant acquiring advanced weapons from friendly nations. The initial main partner in this process was the US, and much of the groundwork of supplying the Germans had been laid by US Undersecretary of Defense Frank Nash during the EDC debates. The Nash List provided the Bundeswehr both surplus M47 and M48A3 tanks under the Military Assistance Program (MAP) as well as other sophisticated equipment like F-86s. The Bundeswehr also sought to fill in gaps by tendering orders to Western European states. This not only had the effect of making German rearmament appear "European," but also eased the FRG's trade deficit with Western Europe, which had grown with the export-orientated Wirtschaftswunder. Thus the new Bundeswehr was armed with American heavy equipment, but French Alouette II helicopters, Belgian small arms, Turkish ammunition, and Swiss-designed and British-built Hispano Suiza HS-30 IFV. Israel too was a beneficiary of Broad Armament providing the new German army with mortars and Uzis (built by Belgium).

But this heterogeneous mix of weaponry was not an end onto itself, but a means for the Bundeswehr to familiarize itself with modern equipment in order to fulfill "deep armament." This policy stressed that FRG defense firms would gradually take over the Bundeswehr's needs and provide its military chiefs with the equipment they wanted. The Germans, most of whom were veterans of the Second World War, were disappointed with the quality of MAP weapons and the US's poor performance in Korea did not make American methods ones to emulate. Blank had tried to get British Centurion tanks, but the British were alarmed that the Bundeswehr was not becoming an infantry-focused defensive force, but was instead developing a strong armored component, and refused. The British concerns proved apt as the Bundeswehr sought with deep armament to create the type of force structure that reflected what it saw as the lessons of the war. Both Blank and Strauss, as well as the new service heads Spiedel and Heusinger, envisioned a mix of modernized Panzer and Panzer Grenadier formations to be the bedrock for the Bundeswehr. This meant, for example, developing the IFV concept for mechanized infantry and abandoning the US's concept of a battle taxi. Even though the HS-30 was a substandard piece of equipment, it did provide practical experience that went into the design of the Marder IFV. Deep armament also called for the Bundeswehr to be nuclear-capable, with fighter-bombers and artillery, later missile batteries, capable of deploying nuclear weapons. This issue was too much for other NATO partners to completely stomach, as well as quite unpopular at home, and while the Bundeswehr did evolve a nuclear-capable force, the warheads were strictly under NATO (ie non-German) control.

As the Bundeswehr's nuclear capability demonstrates, German rearmament was still a political hot potato. This had implications for the development of the first generation of MBTs. Strauss, who hewed towards a Gaullist grand strategy in contrast to the more Atlantic-orientated members of Adenauer's cabinet, proposed a joint Franco-German development for a modern tank in 1960, the Standardpanzer30. This was part of Strauss's larger game of building up a closer Franco-German rapprochement, which could have included a French nuclear weapons in exchange for a FRG withdrawal from NATO. The resulting competition had one French firm producing what would be the AMX-30, while two German teams worked on the proposed German design. The two teams were headed by both the Porsche and Henschel firms respectively and included many veterans from those firms' wartime design and production teams. The Porsche design eventually won out over its French and Henschel rivals and became the Leopard I.

As the above indicates, West German rearmament was not just simply a case of picking up where wartime development left off. Instead it was a tricky process for the Bundeswehr as it sought to navigate various political tripwires to achieve the type of force structure the Germans actually wanted. Fielding a tank force was not just a matter of damaged German industrial infrastructure, but making such a reborn force palatable to the FRG's neighbors to the West. Wartime lessons had underscored the need to have a modern and technologically capable force and despite the Wehrmacht's reputation for scientific Wunderwaffen, many of their much-vaunted equipment suffered from bugs and defaults from too hasty of a development cycle. By the time the political constellations had formed allowing for an indigenous German heavy tank, the modern battlefield had changed so much, especially with the anticipated NBC environment of a war in Central Europe, that a tank designed in 1960 would be a very different animal than one designed in 1943.

Sources

Birtle, A. J. Rearming the Phoenix: U.S. Military Assistance to the Federal Republic of Germany, 1950-1960. New York: Garland, 1991.

Corum, James S. Rearming Germany. Leiden: Brill, 2011.

Large, David Clay. Germans to the Front: West German Rearmament in the Adenauer Era. Chapel Hill: University of North Carolina Press, 1996.

Wenger, Andreas, Christian Nuenlist, and Anna Locher. Transforming NATO in the Cold War: Challenges Beyond Deterrence in the 1960s. London: Routledge, 2007.