The PACs. I think this practice should be considered blatant corruption in any democratic system as it enables large corporations and wealthy individuals to predetermine which candidate or party has even the slightest chance in elections. In my home country, of course, there are private political funds as well but those are not nearly as important in our system as there is solid public funding for political parties based on past election results. I might be wrong but I always thought that the insane amount of private money that fuels US elections boils down to the US being a plutocracy rather than a democracy.
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”.