Sorry to add yet another answer **but**...

The answer is simply that politics makes for strange bedfellows.

It's harder to campaign on your platform, because people who are nominally "on your side" are not going to support every aspect of your platform. It's *much* safer to lambast your opponent for the things that made the people on your side be on your side in the first place.

By way of example, how much do Hillary and Bernie supporters really agree on beside how much Trump sucks? I mean sure there are some things that anyone on the Left are going to agree on, but the fault lines are there and it makes way more sense as a political strategy to paper over them by rallying around hating Trump than having them register on the Richter scale the way they did during the last election.

Same thing plays out on the right where you have an increasingly uneasy coalition of military-industrial complex neocons, religious fundamentalist tradcons, tea partiers, and (sometimes) libertarians. Oh, and that one guy who doesn't really belong in any of those groups...

The things you focus on are going to be the things everyone on your side agrees on, and those will be almost definitionally be the things that differentiate Right from Left in the broadest possible way, and it's easier to rally around those values by deriding your opponents for not having them (or for giving them less importance than you consider their due). 

You can also take a controversial view that some people on the other side hold (e.g. communism for the Right, homophobia for the Left) and try to tar entire opposing side with that brush more easily that you can get your own side to rally around your own platform.

In our FPTP system it's the best way we've found so far to maintain party unity among diverse groups.