I think one factor is that Democrats and Republicans actually hardly differ in many fundamental positions. I think the fact that an unscrupulous business man like Trump, who was once a member of the Democratic Party, can switch party affiliation just like that illustrates what I mean: there are no real alternatives, which is why election campaigns in the US need to be emotional rather than rational. That favors baseless fear mongering and empty finger-pointing that misses the real problems. I supect that many US citizens have become so accustomed to these empty election campaigns that they lost the ability to identify the lesser evil in this charade of mutual accusations far away from rational discourse. So in short I think Trump was just the better demagogue which is pretty much all that matters when reason or actual arguments are not part of the “election show”.