It might be because some ASD traits help people become more successful in the information economy. Asperger's people often thrive in well-paid occupations such as programming and data analysis where spontaneous social interaction is widely acknowledged to hurt productivity.
Or it might just be a side effect of a society that places a lot of emphasis on intelligence, making it more likely for a person with high IQ to seek a mate with similarly high IQ. The more smart people "inbreed" among themselves, the more ASD cases we will naturally see.
In either case, since evolution is whack-a-mole, this will lead to not only mild ASD but also more cases of severe autism if they are caused by the same set of genes.
I do believe that children with Asperger Syndrome often see the symptoms grow less severe as they age. Perhaps with lots of hard work and a good support structure, some of them people grow to enjoy above average success. It seems entirely unreasonable (and to my mind somewhat cruel) to credit this success to Asperger Syndrome itself.
http://www.npr.org/sections/health-shots/2015/09/02/43674237...
An allergy or intolerance is much worse when it make your nonverbal child irritable and violent, then when it makes your verbal child say "mom, my tummy hurts bad" after you serve pizza.