The L7 was developed after the 1956 Hungarian Revolution - Hungarians delivered a Soviet T-54A to the British by literally driving it into their embassy. After they examined the armor, they (the Brits) realized the 20 pdr wasn't going to be sufficient in either range or penetrating capability against the new Soviet designs, and the 105mm L7 was born. And the Lord saw it, and it was good.

Meanwhile, in the US, the M48 was still fitted with its 90mm gun - not a bad weapon, but when the British reports on the T-54A reached US designers, they knew the venerable 90mm wasn't going to cut it. They settled on the British design as it was already sufficiently developed to counter 2nd generation MBT armor.

The M60 Patton was designed to use the new gun - basically an uprated M48 chassis with new gun gear. The L7 hung out in the US inventory until it was replaced with the M256 120mm (a German design) in the late 80's when the Abrams went to A2 specifications. Reason for the shift was an increase in Soviet armor efficacy that a rifled 105mm gun would not easily counter.

I wouldn't go so far as to call it revolutionary. It was simply a quite conventional piece of artillery with the required performance. High velocity,good accuracy and a fairly compact breech. What helped it stay in service for so long was the development of improved ammunition, particularly the fin stabilized sabot round made of depleted uranium. This enabled the same weapon to fire a projectile with far greater penetration than simple APDS and HESH rounds used previously.
As for why it was adopted by so many countries, It makes no sense to develop a weapon at great investment in money and time when there is a suitable solution available off the shelf.

There was a lot of advanced ammunition available for the L7. By the 1950s- when war was expected to start any time- designing a gun meant also years to develop a range of ammo. The L7 was not especially revolutionary, being, to be slightly cruel, a bored-out 20-pounder, but after spending the whole of World War II with rubbish tank guns the British had learnt a few things about ammunition design while everyone else was just trying to produce ever-larger guns. The APDS and later APFSDS rounds, and unique-to-Britain HESH round, could take care of the heaviest enemy armour, while canister, HE and WP rounds could finish anything not worth using AP on.

The L7 was an ideal size- it could fit easily into a medium tank, and was big enough to see off almost anything at effective battlefield ranges. This combination of Goldilocks-pleasing size, and ammo options, made it a real winner.

In the 1960s various nations went off on tangents with tank gun design: the US with the Shillelagh gun-missile, the USSR with the sooth bore 115 used on the T-62. In the end, these guns generally proved less capable than the good ol' L7. The smooth bore gun used fin-stabilised rounds that were inaccurate in crosswinds, the Shillelagh presented the issue of the massively-expensive wireless guided missile being much more expensive than most potential targets. It's bagged rounds proved hazardous, cordite dust collected in the bore.

Most of these problems were overcome to a greater or lesser degree, but meanwhile the L7 was saving Israel's bacon and steadfastly refusing to become obsolete, in part because the projected super-heavy tanks of the 1950s all proved abortive and irrelevant.

Other overarching issues were the constant desire for NATO standardisation (Korea proved a logistical nightmare) which especially affected tank design (multifuel engines for example). The MBT-70 project fell apart mostly on standardisation issues, giving the L7 another reprieve. By the time the L7 was going into the first generation Abrams, there was a distinct feeling it was becoming obsolete, despite it proving itself yet again against the then-new T-72 in the Bek'aa Valley in 1982. The British had already designed a new 120 for the Chieftain (while keeping lots of 105 Centurions on hand), refusing to be distracted by the missile craze of the 60s. The US and Germany finally learnt their lesson.

In Tank vs Tank by Macksey it says the M48A2 still had the 90mm gun "due to American fascination with a projected smooth bore weapon which unfortunately failed (expensively) to come up to specification." Is this true? Does anyone have any information on the failed tank gun?