Even if Russia used nukes to the point where Ukraine surrenders, there's still the question that "now what?" Russia would still need to occupy all regions they want for themselves and we're way past the point where they could install a puppet government and expect Ukrainians to obey without the threat of violence. The mad thing for Russia is that at this point even a victory would bring very little benefit to them as a country, mostly just personal benefit to Putin and his cronies.

Then there's the negatives about using nukes. A very possible military response from the USA. Even more sanctions. Very possible that all Russian-owned real estate and other property in the West gets confiscated. A lowered threshold for using nukes against Russia. More isolation and even their only important friend, China, would likely backtrack on ties.

The negatives hugely outweigh the benefits. Only Putin benefits in the sense that he could claim victory if Ukraine surrenders. It's also possible his order of a nuclear strike would not be carried out and he would be stopped by people who don't want Russia to face the sort of isolation and economic damage that would follow.