Because Baltic states were part of Russian empire once and USSR later. Putin would be forever glorified in Russia and could rule till the end of days if he'd manage to "restore the glorious Russian Empire". Another reason is that Baltic countries stand as a gate to (old) Europe for Russia in both economical and military meaning.
RE: "restore the glorious Russian Empire" - his latest customs/economic union with Kazakhstan and Belarus is a joke. And there's two most pro-russian regimes as of now. CIS is also as good as dead. USSR is gone and won't be restored.
How Baltic countries gate from Russia in "economical meaning"?
Because Baltic states were part of Russian empire once and USSR later. Putin would be forever glorified in Russia and could rule till the end of days if he'd manage to "restore the glorious Russian Empire". Another reason is that Baltic countries stand as a gate to (old) Europe for Russia in both economical and military meaning.