Russia is the aggressor. What do you propose?
They have a large nuclear arsenal; I'm sure the news would have covered it being used.
What news do you follow that shows Ukraine as a hot nuclear desert?
Reason is simple - using nukes when you can't win a conventional war will degrade them from strategic asset into tactical one and whole nuclear proliferation flies out of the window and everyone will have nukes.
Have you seen the Oryx list of destroyed equipment? The defunct Soviet Union produced a shitton of armored vehicles, Russia inherited most of them and they all got burnt in Ukraine.