Jay Rosen explains that Donald Trump's vast wealth makes him immune to the forces that normally confine presidential candidates.
But notice: Trump is not an institution. trumpairHe is really his own campaign manager, spokesman and chief strategist, which means that the chief strategist of the Trump campaign -- Trump -- doesn't care if he ever gets hired by another campaign. Poof! There goes one of the little structural forces that tend toward isomorphism. Multiply by 100 and you have pundits asking: have the laws of political gravity been repealed?
Rosen has several other good points, but I think they all come down to the fact that Trump's wealth makes him willing to alienate powerful/important people that other politicians want to be liked by.