1) Very true, there are better means and incentives to diagnose conditions now, as compared to the old days, creating a perceived increase.

2) I've also read another compelling contributor: There has been a tremendous area of genetic narrowing due to increased educational opportunities (and associated marriages among diverse sources of intelligent people). Colleges fielding smart people the world over didn't exist until maybe 80 years ago. Colleges "select" according to highly focused individuals, whom we might also describe as partially autistic. When college people have offspring, this tendency may be genetically reinforced.