Trump's popularity is due to a few factors I think ...
1) Many Republicans are fed up with and distrust the establishment of the party that has consistently failed to live up to the rhetoric they campaign on. Some folks seem to have become so fed up, they'll back anyone who will take on the establishment - irregardless of their actual positions.
2) Trump has definitely tapped into the angry anti-PC vibe. In years past Trump being a vulgar, petty, thin skinned, playground style bully would've been an automatic disqualifier to most of the public, but a certain segment celebrates that because its "not PC" and Trump is just "telling it like it is". Furthermore, the Republican base has increasingly wanted a "fighter" as their standard bearer. Not a moderate like McCain or Romney. Heck, 4 years ago John Huntsman had rock solid conservative credentials and was widely panned as a RINO for little more reason than the tone of his campaign which was thoughtful and reserved, rather than antagonistic and aggressive. Trump's tactics are what some people consider to be the tactics of a "fighter".
3) Trump has also brought in a lot of non-Republicans who were traditionally apolitical. These folks have bought into the one part of Trump's message that has been fairly consistent - an America first, anti-trade, isolationist, anti-immigrant message. Protectionism and isolationism are two things the establishment of both parties reject and thus people who feel this way have few outlets to back.
