There are a couple of reasons for this. The first is that Russian tactical nuclear warheads have been in storage since the 1990s and nobody knows if they have been properly maintained or serviced in the intervening period. In fact, nobody knows if they will still work, and some commentators have said that they are now probably more dangerous to their owners than they might be to anyone else.
The second reason is that the warheads are not actually attached to their delivery systems – missiles and artillery shells – and have to be outloaded from their depots and bunkers and married up with the aforesaid delivery systems before they can be used. And the minute this process starts the Americans and NATO will know via their all-seeing eyes in the sky. The USA has already warned Russia of terrible consequences should such weapons be deployed, without being specific as to what that response might be.
In short, Russia has little to gain and everything to lose by going nuclear. The one great unknown is the state of Putin’s mind; will he realise the perilous state he is getting Russia into, and does he care anyway? If he decides to go completely rogue and deploy his nukes, are their saner and wiser heads in Moscow who have the courage and wherewithal to intervene before he acts? I don’t know the answer I’m afraid, nor does it appear that anyone else does either. We are well advised to keep up our guard.