Because divergence creates natural out-groups to model your derision after.

Even then, it really just boils down to perspective. I've seen terms like "breeder", "redneck", and "normie" used as slurs. Any out-group will do. Medical terms tend to imply weakness or dysfunction which just happen to attract bullies like flies to honey. But then so does sexual proclivity (or the lack thereof), racial and gender traits, academic performance ("dropout"/"failure"/"loser"), economic standing, fashion sense, preferred hobby, political affiliation... you name it.

In internet forums, where people tend to pride themselves on intelligence and social skills, neurodivergence just becomes the most obvious characteristic to attack.