In a nutshell, this is why Trump's their guy

Trump is the candidate who finally figured out how to exploit the fact that much of the Republican voter base does not share the policy preferences of the Republican donor class, and that it is therefore possible to win the nomination without being saddled with their unpopular policy preference.
The Donald ran away with the nomination in a party that has been dominated by Big Money, Far Rightwing Ultra-Conservatives. Using Citizens United Money and Bullshit Mountain, (and aided mightily by Bible Belt preachers), the monied elite have hi-jacked the once traditional party and turned it into something so far Out There that their own St Ronald would be considered a flaming Liberal. Trump recognized that and used his TV recognition and racist bigotry, appealing to the 'ill educated' as he called them, while being loud and crazy enough to dominate the media. And the sheep ate/eat it up. Of course there's no way he can win, cuz the general population isn't gonna fall for his crap, but it was enough to hijack the GOP, which is good for the country in the short run and maybe good for them in the long run, cuz they are rapidly/rightly going extinct, while the Koch bros, Big Oil, Big Military and all types of Big Money have sculpted it to their desires, with ample help from FOX/PARP and the Bible Belt sheep. Blatant fear-mongering has kept the sheep cowering from ISIS, Muslims, immigrants and all types of 'demon possessed' gays, while the real enemies of the middle class, the 1%'ers, take all the economic gains.
In a party dominated by those who think America went 'to hell in a hand basket' when Elvis was replaced by the Beatles, it's no wonder the young and educated are overwhelmingly non-Repub.
Bottom line, MAYBE the Repub party can return, after a thorough cleansing and there may be a third party, (which might be a good thing), but hopefully/thankfully, the Repub party we have come to know and hate will be dead and buried, as they should be. GO DONALD!