Crimea has the naval base at Sevastopol. Russia’s Black Sea Fleet is based there.
The Donbass on the other hand, hasn’t been annexed because its smokestack decrepit economy would cost a fortune to rebuild.
Putin is many things, but being an idiot ain’t one of them.
Yes, the Fleet is at Sevastopol,so it makes strategic sense. But it doesn’t change the fact that Crimea is a money pit.