No neighbouring country would want to add a population centre with a million Russians to its polity because then you'd have a million Russians agitating for closer ties to Russia, if not actively facilitating or planning a coup.
Lithuania, to the north, has a population of just under 3,000,000, of whom Russians are 5%. Take Kaliningrad and suddenly they would be 30% Russian.
Poland is 37,000,000, so Russians would only be around 3%, but that's enough to cause a lot of trouble, and they gain nothing they don't already have besides a ready-made fifth column.
Lithuania, to the north, has a population of just under 3,000,000, of whom Russians are 5%. Take Kaliningrad and suddenly they would be 30% Russian.
Poland is 37,000,000, so Russians would only be around 3%, but that's enough to cause a lot of trouble, and they gain nothing they don't already have besides a ready-made fifth column.