It's a slur, that's why. Some slurs are major slurs that are considered offensive even when it's not referring to someone, and I think this is one of them. They're often forbidden because of it, and a lot of people find it easier to follow universal rules than to check the rules of each place they post, because the universal rules will keep you in the clear 95% of the time. And even if not, it's still respectable to not want to use slurs.

And yeah, it did used to be a lot more common. And before that, it was a medical term. That's the way it goes, we have a word to insult people's intelligence, doctors deal with patients who have a disorder causing that, they don't want to use the mean word to refer to the patients, they come up with a new word for it, the word gets out, and then everyone makes it a new word to insult intelligence. Though this one seems to be taking a different track because it hit modern sensibilities, and at the same time I think doctors now know better than to come up with a new word for mental deficiencies.