Maybe because while it's a disability and the root of that disability is in a developmental problem, it's not so much a developmental disability as a developmental abnormality. I think "developmental disability" generally implies mental retardation. (And, granted, the term "developmental retardation" might actually technically be applicable because people with ASDs mature more slowly and "ret*d" means "slowed," eve that term may somewhat apply even when in someone with above average intelligence..)
Um, I'm not sure that this was making sense.. I have a thought that kinda makes sense, but I'm not sure it's coming out right. The words of term can be technically correct, but if those words are more often used to describe something different, then using them is misleading, because it implies the other issue rather than the simple denotative meaning.