Almost every Republican-leaning commentator predicted Romney would win. Krauthammer, Gingrich, Morris, etc. I believe the polls were leaning Romney as well. Only a minority of polls predicted the minority-heavy turnout that Obama had in 2008 would repeat again.
Trump has the unique albatross around his neck of the #nevertrump movement, which Romney didn’t have to worry about. He does not have the party fully united around him. While #nevertrump is much less a grass-roots movement, and much more of a top-down movement from the elites, it’s still something we’ve never really seen a presidential candidate face in a general election before. It seems a bit optimistic to think that support from the party’s base will be unaffected by this movement being backed by this many regulars in the “right-leaning” media. Trump has to hope for a “Brexit effect” of a new swath of voters turning up who don’t usually vote that’s big enough to counteract it.
Remember the primary numbers.