I suspect that part of the reason more of us are diagnosed with autism is that we are the kind of people who seek therapy for other reasons. That puts us in a position to be diagnosed.
When I was attending school for my certification in hypnotherapy, we were told that, overall, more women seek out therapy than men, and that the difference was not about biological sex. People of both sexes with an emotional personality type were more likely than those with a physical personality type to seek out therapy.
People who avoid therapy rarely get diagnosed, and those who receive therapy do. I suspect the numbers are higher because of the number of people who have it, but just never received a formal diagnosis. In the same way, there are many more transgender people than the numbers tell us because of the number who remain stealth, or are undiagnosed, or for whatever reasons.