It's one of those unofficial community rules which is tragic and really funny at the same time. Like what kind of boring and ultimately cold community do you get when your community policing doesn't allow humor?
> I agree with what people have already said, but I think there's one more point to add: people usually over-estimate how funny their own comments are. We have a tendency to think, "This idea of mine is hilarious! And different! Surely this witticism is the exception." And we are usually wrong. When you have N people all doing that, there's a lot of noise.
> I try to gently point this out to people who complain when their attempt at humor has been downvoted by the community. It's not that we don't like humor. We just don't like banal attempts at humor, which becomes noise. Or, put in a less charitable fashion, "You're not as funny as you think you are."
Humor is allowed, but low effort memes and cultural references tend to crowd out interesting substantive discussion and are so predictable as to not add much value. Look at popular Reddit threads, you can often guess the top voted comment.
This community is not meant to reflect an average of the Internet, it's specifically intended to be a place to expect original and intelligent discussion - including the humor.