I think the short answer would be that Ukraine is too close to Russia for Russia to even consider using it's most deadly weapons on them.  Sure, Russia could nuke Ukraine off the planet, but it would certainly have side effects for their own home as radiation would surely have side effects on their citizens.  Not to mention they would absolutely destroy the land they're trying to take.  It would make no sense whatsoever.  This isn't even touching on the response from other countries.