The Soviet government constrained the use of their nuclear arsenal for defensive purposes only, but the government of Russia today operates under no such legal compunction.
It's worth noting that unlike the Soviet Union, the United States has never agreed to 'no first use' and that to this day, a preemptive nuclear strike has remained part of American military doctrine (and has been since WW2).
Current doctrine allows us to use nukes to "ensure US and international operations are successful". Not that this excuses Putin's aggressive posturing at all of course, but sometimes I wonder if global nuclear nonproliferation/disarmament efforts would be more successful if every nation could simply agree that they wouldn't start a nuclear exchange
