An idea that I had:
Some decades ago, children with moderate social problems were identified relatively late: until 6-7 y.o., they stayed at home; in the first school years, any problem was considered "the adaptation to the school"; then, only in the late childhood/begining of adolescence is that there problems are really noticed - and, at these age, any problem was considered more psychological then neurological (perhaps even the begining of adolescence crises).
Then, there was few specialists working in the field of "innate/neurological social problems in children", then, few people people to translate Asperger's papers.
Today, children go to pre-school relatively earlier (perhaps at 1 year age), then this kind of problems gets much more attention.