I’ve never understood why people can’t just play the game they like and let other people (and in the age of the Internet, the other people are often people they will never even encounter IRL, much less play with) play the games they like. Unfortunately there are all sorts of groups online dedicated to telling each other why everyone that plays differently than they do are Dumb And Wrong. Some people want to hate on things that are popular and some people get angry at newcomers to their beloved hobby and even others get angry at any perceived “dumbing down.”
No one is coming to confiscate your Rules Cyclopedia or copy of OSE. Live and let live. If everyone at your table is having fun, it doesn’t detract from my or anyone else’s fun.
The problem with some people is that, whether they think of it consciously that way or not, really expect they're in a struggle for mindspace.
People have, at some point, almost always had trouble getting players together for a game, or notice that they're not getting additional material for the version of the game they use. And the take-home they get from that is that people who want different things out of a game are a passive threat to their enjoyment.
Its hard to even call it completely irrational. Ask anyone who's wanted to get together a game of something other than the current edition of D&D in certain areas how much differences in taste doesn't matter.
But the reality is, getting into a death battle about it is pointless. The fact you're telling people their fun is bad and wrong isn't going to stop them mostly, so what are you doing other than cooking up stress?