The “entrance” polls didn’t match the results
Something weird happenned in the Caucus, very Nixonesque
Polls don’t mean much when turnout is what decides outcomes. That’s especially true for caucuses, which are a bigger time commitment for voters than primaries. Cruz’s evangelical base was more committed to get out the vote than Trump’s supporters. It’s the same reason that Rick Santorum and Mike Huckabee managed to win Iowa in 12 and 08: their supporters weren’t necessarily higher in number, just more committed to turn out and caucus.