I understand not calling disabled people the word, because mocking people for something about themselves they didn't choose (like a disability) is cruel, I am totally on board with never using words in this way to target disabled people.
I don't understand why I can't use the word to mock someone who is not intellectually disabled for choosing not to use their perfectly well-functioning brain, it seems like a very apt analogy. It communicates "you aren't disabled, you have no excuse for acting like it, start choosing to use the fully functional brain you have".
Additionally, only the "r-word" seems to be the bad one, despite there being many other words in our language that originally began as a medical descriptor for intellectually disabled folks. If I call someone a moron for running a red light because they're playing with their phone nobody bats an eye, but if I call them the "r-word" I'm a terrible person?