Because as the Republican party has moved so far to the right that even the kooks that used to populate the party need a home, they are moving to the Democratic party. While they do this, they are pulling the Democratic party even farther right than it already was. It wasn't too long ago when the current Democratic platform would have been a comfortable right-wing platform for the Republican party. Now? It is "Dangerously Socialist" to the average Republican politician. If only their voters see that putting them back in office is against those voters' best interests. But that involves American voters being aware and involved beyond talking points and rallies, which means it isn't likely to happen.