I guess a big factor must be to ask what they would gain. Domestic PR / distraction is one thing, but Crimea also has warm-water ports, and a (largely?) ethnic Russian population.

Eastern Poland has... some farmland, some ancient forests? People with a 5-generation deep tradition of suicidal attempts to kick the Russians out? Whereas the Baltics are sandwiched with Kalinngrad, and some have fair-sized Russian populations, who can be painted as oppressed. That seems much more concerning.