It's odd that Trump is so low. If there really is so much dissatisfaction with Trump, he wouldn't have been getting 70+ in primaries after his opponents dropped out.
What kind of turnout are you actually getting in those late primaries, post-Cruz/Kasich dropout though? I would imagine that there would be plenty of people who would have preferred a different nominee who nonetheless aren't going to bother to protest vote after the nomination has already been decided. For that matter, you could have voters who would have preferred another candidate yet still vote for him once the nomination is decided because "he's better than Clinton", and they want to rally around the flag.