The Republican mostly-winner-take-all primary delegate system, and lack of superdelegates, that's what nominated Trump. He never had a majority, but his plurality was enough.
That couldn't have happened with the Democrats, due to the way they allocate delegates proportionally, and because they have so many superdelegates. If the Republicans did things proportionally, the not-Trump contingent could have combined and nominated a single non-Trump candidate.
Trump had the most delegates in a large pack of candidates.
It is the same reason that McCain won.
I don't recall any conservatives whining about McCain not having a majority of delegates back then.
I personally blamed Huckelbee for sticking in way too long.