Maybe forcing a mechanical failure provides more plausible deniability than simply blowing it up? It also makes it easier to repair considering it's still a valuable source of water for Crimea.
A hydrologist on twitter said that it would require far fewer explosives to blow it up when overfilled. If they were short on those, it would be another reason.
Easier to repair? Did you see the pictures? Not an engineer, but that looks like they will have to tear down whatever is still standing and rebuild from scratch.
