Obscurity may also have something to do with it, but if I'm remembering right Asperger's was comparably obscure until at least the late Nineties.

Actually, what's now called Asperger's was initially part of what Kanner called autistic psychopathy. However, some people with severe problems and/or mental retardation also had the same symptoms, so the diagnosis was expanded to cover them. Then it narrowed to include entirely those with very severe disabilities, such that autistics/Aspies with the ability to "pass" (act normal or act like something other than disabled) to any degree were overlooked despite needing recognition, information and assistance. So a researcher decided to introduce a new diagnosis, Asperger's Syndrome, to cover spiffy-shiny-cool autistics, as opposed to need-lots-of-help autistics, because the spiffy-shiny-cool kind still needed assistance and often had serious problems as a result of the mismatch between their abilities and other people's expectations. Asperger's entered the DSM-IV in the early/mid nineties.