Russia has tried to justify its invasion of Ukraine in terms of getting rid of an undemocratic (and neo-nazi) government forced on the country in a coup orchestrated by the west. There is also talk of protecting the interests on ethnic Russians inside Ukraine from government oppression and Putin's claims of genocide against those ethnic Russians.
I don't think that even Putin actually believes all those claims, but that is what is being used to publically justify the invasion.
Nuclear weapons are obviously not the right tool to use if your goal is to get rid of a government in the name of protecting the population. It would cause massive civilian casulaties, destroy critical infrastructure, and make major cities uninhabitable for some time. It is impossible to square that with trying to defend the Ukrainian population.
If you subscribe to the western view that Russia is trying to bring Ukraine back in to its sphere of influence and be essentially a client state, nuclear weapons still don't make any sense. Russia wants to bring Ukraine's manufacturing, economy and military (and territory) under Russian control. Again, this is best achieved by getting rid of the government while keeping the rest of the country as intact as possible. A functional Ukraine subservient to Russian interests is a much better outcome than a broken Ukraine whichever way it is aligned, so that is what they are trying to achieve.
Whichever view you subscribe to, Russia has every interest in trying to minimise civilian casualties and impact while still achieving regime change.
The only time that calculus comes in to question is if it looks like Russia won't be able to create regime change through conventional military means.
Or to put it another way, this isn't a game where the point is to "win" the war by "beating" the opposition, and then getting the game over screen. In the real world the war is "won" by achieving the political objectives behind the war. Nuclear weapons do little or nothing to advance those objectives, and come with massive and unknowable downsides.