His opponents by and large were pretty weak, and there were too many of them. I don't know if there's a practical way to limit the field though.
Another issue is they didn't take him seriously early enough, and I'm still wondering why there hasn't been much digging into all his shady dealings over the years, and his blatant lies about his business record and finances. Then there are suspected mob ties, exploited workers, all the infidelities, etc. And, Priebus is weak and feeble and should be taking more of a leadership role to keep this pos from getting the nomination by any means necessary. Overall it's been a lot of hand-wringing but no coherent, coordinated effort to take him down.
What makes me ill about politics in general too -- all the congresspeople who won't take a stand one way or another, though they clearly despise him. If you had dozens of them stand up and say "I'll never support Trump under any circumstances", maybe that would resonate. Christ, even weaselly Cruz won't vow not to endorse him. This crap is why everyone hates Washington.