My best guess is lack of knowledge. In a lot of places there just aren't many people who specialize in autism in general, and the ones that do...well, I'm betting they know about autism with children and not so much adults. For a long time now, autism in adults hasnt really been represented well. It's like we vanish once we turn 18 and get out school or something. The focus is usually on kids. And if we don't get that diagnosis as a child, it seems it would be harder to get that dx as an adult, I dunno. A knowledgeable clinician would help, but again, how many are there? Some people luck out with their professionals, others not so much.