It's not clear exactly what you're asking.

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There absolutely was nuclear artillery developed during the Cold War intended to give heavy artillery units the ability to deploy tactical nuclear weapons. The "Davy Crockett" recoilless rifle concept is perhaps the most famous, but over the years tactical nuclear warheads have been developed for everything from the 16-inch guns on the Iowa-class battlehips to 203mm and 155mm field guns. These would be complemented by short-ranged rockets and strike aircraft to deploy nuclear weapons on the tactical level.

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If you're asking why this wasn't done in the immediate aftermath of WW2 to stop the Soviets, then you've got a couple of problems with your premise. First of all, the Western Allies absolutely did have air superiority at the end of WW2. While the Soviets would quickly address the problem with the development of jet fighters, the Soviets at the end of WW2 completely lacked an effective means to intercept high-altitude bombers like the B-29. They had no domestic fighters capable of intercepting the B-29 at altitude, and the aircraft they used for high-altitude interception during the war (Lend-Lease P-47s and Spitfires) were few in number. Because of that, in 1945 and up until the Soviets had significant numbers of jet fighters in service, B-29s would have been able to hit strategic targets in the USSR with relative impunity.

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That being said, there's also another issue with quick deployment of nuclear artillery - size. Nuclear weapons were still in their infancy in 1945, and it would take several years for the technology behind it to advance to the point where it could be shrunk to a size that would be practical in an artillery shell.

The US began developing tactical nuclear weapons, including nuclear artillery, in the early 1950s. The main spur for this came from Project Vista (1951), an Army-sponsored project that meant to evaluate the role of atomic weapons in the defense of Europe against the Soviet Union. It recommended, in short, that low-yield, flexible, tactical atomic munitions be developed and deployed to counter tank formations and the like. This was, it should be noted, in contrast to the previous (and Air Force) philosophy of nuclear weapons, which emphasized high-yield, strategic weapons that would threaten cities and centers of production. Which is part of the answer to your question about why this took awhile: aside from the technology needing to be developed, the main technological "push" in the postwar was towards making a larger stockpile and a higher-yield one, whereas a tactical approach frequently means "wasting" then-scarce fissile material in less-efficient arrangements that give lower yields.

This was, as an aside, one of the ways in which J. Robert Oppenheimer developed political enemies: the Air Force took grave exception to Vista, which they saw Oppenheimer's fingerprints all over. Ultimately, of course, the Air Force learned to love tactical weapons as well, but at the time they saw this as a push by the Army to get in on their "turf."

Along with the other misconceptions addressed in another comment, I just want to note that North Korea does not have nuclear-tipped artillery aimed at Seoul. It has conventional artillery aimed at Seoul (in great numbers), and it has short- and medium-range nuclear missiles which may or may not be aimed at Seoul (along with longer-range missiles, too).

Oh yes, the US Army had a couple of different types of nuclear artillery.

The most obvious was the M65 280mm Atomic Cannon, aka "Atomic Annie". Annie was a clumsy sort of thing, a very heavy 11" gun that required two prime movers (one at each end) to transport at relatively low speed. A further nine vehicles were required to carry the gun crew, supplies, ammunition, etc etc etc. Once the 84 ton, 84' long cannon was assembled and put in battery, a process some 20-25 minutes long, it could fire the W9 or W19 nuclear shell out to about 20 miles. A video of the transport, construction, and firing of Atomic Annie is here.

Atomic Annie operated perfectly on its one and only firing of a nuclear shell (Test Upshot-Knothole Grable), but certain tactical limitations became clear immediately. It was designed to break up attacking Warsaw Pact forces, but it would be rather sluggish to respond given the long construction time, not even considering the transport effort required. The other option would be to have it assembled and ready to go, which meant it was totally immobile. In either case, it would come as a surprise to nobody to hear that the Soviet military would be very attentive to the location of any M65 in theatre (there were some 20 made) and given a high priority for destruction. Effectively, Atomic Annie was obsolete the moment she reached the battlefield as nuclear tipped tactical rockets with much longer range had been created. Later, the self-propelled M110 and towed M115 8" guns could fire the W32 nuclear shell. These were at least more maneuverable and thus somewhat more survivable. Their main advantage would be that they're just another artillery piece and thus wouldn't be directly targeted like Annie.

At the other end of the scale was the M-28/29 Weapon System, popularly known as the Davy Crockett. This was a 51-pound M-388 nuclear round fired by a recoilless rifle that could be man-carried or mounted on a jeep. This weapon had an adjustable size warhead, anything from 20 tons to 2 kilotons of explosive force.

It would be the ultimate in "shoot-and-scoot" weapons as the jeep-mounted version could be loaded and fired in just a couple of minutes, and stowed as the jeep pulled away to find a hiding place. The man-carried version can be seen in action here.

The weapon had a maximum range of two miles and was terribly inaccurate. As a result, its main offensive capability was the radiation release from the explosion. With the lowest payload setting of 20 tons, this could be lethal out to about 400 yards, more or less. Which brings up the biggest problem of the Davy Crockett system: when set for the 2kt yield (and probably lower amounts as well), the Area of Effect was actually BIGGER than the maximum range. Yep, the fire team was expected to, in effect, nuke themselves.

The system was fielded in Europe from 1961 to 1971.

The US had nuclear rounds the (W48) for the 155mm howitzer system out to about 1993 that had an effective yield of ~70 tons.

For the most part, tactical missiles began replacing nuclear artillery as soon as they came into existence.

The Soviets had their own nuclear artillery, but that's a reply for a different question!