There has been absolutely no appetite for utilizing nuclear arms, except from the Kremlin which has made thinly veiled threats of doing so.
Nukes are bad, nobody wants to use them.
That's what people have thought. A small yield (.5-1kT) to destroy a Ukrainian army base would not warrant a full-scale nuclear response, but would show the world they are willing to cross that line in the sand. Are the US/EU ready for that?
Things are going to get "interesting".
Or until they run out of anything they can fight with, which is already happening with high-precision ammo and many other weapons.
2 nukes were enough for Japan. I bet for 4 to be sure.