People are quick to claim stupidity because on the surface it certainly looks a lot like stupidity. It's actually just ignorance. Ignorance of a lot of things.
Our system sucks. Everyone agrees on this point EXCEPT for Republicans when their team is winning, at which point they switch from "the system sucks" to "our forefathers were the wisest people to ever inhabit the planet, Oh say can you seeee?" To everyone else, our system sucks, all the time.
Millions of people have no time and/or desire to actually examine the millions of intricacies that lead to the suckage. It's super easy for them to just say both parties are responsible, and that attitude comes with a very rewarding ego boost. For absolutely no work, they get to claim the enlightened badge of honor that Democrats can't claim because they are still sheep. It can't possibly be that some people are Democrats because they have actually weighed hundreds of factors and come to a completely logical decision. And so, their ideal candidate is anyone they view as "not part of the shitty system," whether it be Sanders or Green Party or Libertarian or crappy game show host.
You see, they decided first that Democrats suck, and THEN some of them started looking for evidence to support that belief. Guess what? There's TONS of it out there! Democrats do suck! They are people with flaws, unlike enlightened bothsiders. No matter how many times Democrats do the right thing, it will never overcome the hundreds of examples we can find where they fucked up. It doesn't matter if Democrats do thousands, tens of thousands, or hundreds of thousands of good things, because I have handy links to hundreds of examples of them doing bad things.
They never bother to examine the Republicans. They don't have to. They've already conceded that Republicans are bad, why do any work to figure out how bad? Bad is bad. Nobody is trying to convince them that Republicans are good. Even Republicans know Republicans are bad and they aren't trying to convince anyone differently. They are working hard to convince everyone that Democrats are just as bad or worse, and gee doesn't that tune play nicely with "independents'" own jingle.
Even assuming a bothsider was intelligent enough to understand that there are quite a few shades of grey between good and bad, or even between bad and fucking malicious, they still need to have the time and motivation to actually analyze and examine all the factors that led to a bad decision. On top of that, if they have already decided that both sides are just as bad, the reward for that effort is pain to their tender egos. Much easier to just burn it all down.