Stupidity has a lot to do with it, and you can see this clearly by looking at how popular scam products are. I also feel lack of logic plays a huge part in this as well. Also a major flaw in humans is the "us and them" mentality.

Last thing I'd like to point out, is that dividing the populace up is a common tactic used by those who rule over larger groups of humans. What we are seeing here isn't all that new, it's just that we are seeing it being done in real time, because we all use the internet. Propaganda is the name of the game for most social media sites.