* Someone, a person with family and friends, has to follow orders that could kill them and everyone they know, to advance a war they possibly don't support.
* They have to target the right location, and not some relatively empty area to appear to be doing their job
* The missile has to correctly launch
* A single very anticipated launch has to avoid all interception
* It has to accurately make it to its destination
* All of the parts that have been in storage since the end of the cold war have to have been properly maintained, maintenance that costs the US about $10,000,000 per year each. Any small mismatch in timing, composition of the core due to nuclear decay or humidity and corrosion, and electronics all have to work within microseconds of each other.
To use a single weapon with all these risks, and break a 75 year Taboo against their wartime use, is not something a rational actor would do.
Furthermore, the entire US military would have positive confirmation within 90 seconds of the nuclear detonation, and begin to react according to existing well tested plans.
Next, the US and NATO have a vast array of non-nuclear responses that would effectively cripple Russian ability to project force beyond its borders. The conflict in Ukraine is just the stuff we can put into the hands of those quickly trained, and NERFed enough to avoid deep strikes into Russia.
Also, you have to look at the human factors of those who support Putin internally, they don't want to loose all their spoils of their efforts which are mostly situated outside of Russia.