Some views from this side of the pond:
i) he won because HRC fought him in almost exactly the same way (albeit with more and better mud thrown) as the Republican establishment did;
ii) because the political class that she (and her ilk like Blair and the elder Miliband, and Cameron over here) represents are just not electable now (and won't be for at least a generation);
iii) following on from i and ii - because the political class aren't actually any good at politics, or anything else for that matter;
iv) because his position (that the country was broken) was more believable than hers was - just the fact that he was a candidate proved that.
Also I think the overwhelming media support she had, and all the celebrity endorsements, actually covered up what would have been a far bigger defeat for her.