A whole coalition of groups with different reasons for not voting for Clinton.
I'm just pushing back against the "unprecedented Trump blowout" narrative. So far he's got less popular votes than the loser of the last presidential election (Romney got 60,933,504 votes. Trumps at 59,088,517 with 98% of precincts reporting. He probably won't pick up another 1.8 million votes).
A different argument is that Trump attracted new people to the polls. I'd be more inclined to believe that many of those people are sick of establishment politics and their tactics. But I don't think they are actually a majority of Trump's support.