And I could easily flip the question around to OP. Why would Ukraine blow up their own dam, flooding their own territory and potentially crippling their own nuclear power plant? And making a counteroffensive across the Dnipro river that much harder?
It's not to deprive Crimea of water ahead of the counteroffensive, Crimea's reservoirs are full right now so they've got a year's worth in the tank. That's about the only possible benefit I can think of that Ukraine might have got out of this, and even if it were so it would be a trivial benefit compared to the costs. Crimea's water supply isn't going to make a difference to the actual fight that's about to happen there.