People use it as an insult because it's a great insult. Topical, lots of things you can use it for, and it can really wind people up. People who use it as an insult tend not to be implying that the target has qualities generally associated with learning disability, though. Usually it's used to impute social incompetence, a tendency to overreact or go into meltdown, friendlessness, weirdness, etc. — things that tend to be associated, in the public consciousness, at least as much with autistic people who don't have learning disability as with those who do.
The other insult you mention is no longer a current medical term, so can't be compared in usage with "autistic", which can still be used in its clinical sense, as well as in broader non-pejorative general usage.
BTW I'm not sure how to interpret this aspect of your post, but I feel like you're hinting that a learning disability is pretty much the same thing as being "stupid". If you didn't mean that, then fair enough, but whatever you meant, I want to take the opportunity to say that in modern English, "stupid" is an insult and nothing else. It would be pretty shitty to think that it would naturally apply to people with learning disabilities, and it would also be pretty shitty to look down on someone for having low intelligence.