Look closely at the RNC calendar and you'll see that very few candidates can afford to stay in and no one expected Trump to hang around this long.
As I've stated before on FR, the fringe candidates are there to hold on to the favorite son status of delegate rich states, who will toss their support behind the "real 'favorite son'" when the time comes, that's why they announced so late and everyone wondered, "why is he announcing, he surely doesn't expect to win," ala Pataki, Gilmore, Kasich, and even Christie. They GOPe would have gotten another from CA but there's no one in the GOP out there strong enough to pull any votes.
Trump has certainly mucked everything up!