What aspect of GOP primary rules allowed trump to win nomination?
The main thing was the preponderance of Winner-Take-All primaries, which allowed him to, for example, win 100% of South Carolina's delegates with 33% of the vote and build up a lead which eventually became insurmountable that way.
However, more so than the primary process itself was the pre-primary process in which the Establishment coalesced around Jeb Bush, who then proceeded to flop spectacularly in the election, but yet who was unable to drop out due to his commanding lead in fundraising and endorsements. The murder/suicide of negative campaigning between Bush and Rubio helped him too.
I agree with the first part but the idea that the establishment rallied behind Jeb is a myth. 2016 was a wide open field that left the GOP vulnerable to the rise of an outsider candidate like Trump. This contrasts with 2012 when Romney was the only viable establishment candidate and was able to overcome a divided opposition. Most establishment politicians never endorsed a candidate and the establishment did not rally around any candidate, Jeb or Rubio, as they had in previous years to candidates like Romney and McCain. Bush was merely one of many candidates and getting 10-20% at his best was never a great position for him. The open field was the reason Jeb or other candidates didn't drop out as they all presumed Trump would crash and then they could all have a shot. The establishment nor the establishment vote ever coalesced behind one candidate-the closest they came was at the very end with Ted Cruz(!) and while that gave him his Wisconsin win it was too little too late. A split field was a key factor in helping Trump win, the lack of a frontrunner and not the presence of any flawed one.
http://fivethirtyeight.com/features/jeb-bushs-path-to-defeat-began-a-year-ago/
