Why can they not use firebombing and other conventional munitions? If they can deliver a nuke surely they can deliver a conventional warhead. That is enough to level an urban area as we see in Gaza.
They toss bomb from miles back.
Flying conventional bombers over enemy cities requires the ability to replace most of your bombers per year, or air supremacy, neither of which Russia has even a hope of doing.
WW2 was industrialized the likes of which nobody has ever seen again.
Russia hasn’t captured Kiev, their artillery can’t reach that far and they don’t have air superiority - Russia hardly has an airforce anymore.
Buildings in Kiev aren't made out of wood. Firebombs would do very little damage.
Spending untold billions of dollars and tens of thousands of lives for a pile of ash would never pay itself off.