Because Wesley Crusher was written to be seen as an annoying kid who got in the way. That was the social slant of the time. But you watch those episodes now and you realize, "Hey, he was actually very smart, just trying to help, and he even saved the ship a couple of times." So all the "Shut up, Wesley!" moments became so stupid, even if he did deserved it sometimes, that the community decided to lean into it ironically as an in-joke.