Well it was at first. Even after Japan, the US Airforce considered nuking Korea, but were eventually convinced not to by the Marines and Army who argued that deploying nuclear weapons risked a similar move by the Soviets on the battlefield where soldiers were centralized. Remember that the nukes back then were much, much smaller.

Also, a bit of a nitpick, but we do use nuclear weapons. We have for the past 70+ years. We haven't deployed them since Japan, but their role as a deterrent is a form of use.