Probably to attempt to maintain the illusion that your vote matters for their base. Always have to have a boogeyman to point at.
That's completely circular. Begin from the faulty premise that your vote doesn't matter when it really does. Otherwise, neither party would attempt to build & maintain their voter base. Repubs wouldn't thwart attempts by Dems to build theirs.
It's also completely dismissive of the fact that voting in this country is highly skewed along racial & economic lines.