We kind of know why they don't vote in many states that fall into the red column, or are leaning that way: Voter suppression efforts. The ridiculous notion that somehow people are voting fraudulently, so they demand ID's, then close the DMWs in places where most of the Democrat votes lie, purge the voter roles when someone doesn't include an apostrophe or a comma in the right place while registering. Don't send enough voting machines to certain high-Dem precincts.They will if they feel they are represented. Just being democrat (or republican) doesn't do it.
After a while, when the same right-wing nutjobs keep getting elected, a segment of the electorate figures it's no use voting if the deck is continually stacked against you.