Two reasons...
1) The definition of autism has changed to be more inclusive.
2) Environmental toxins.
The definition has widen to include a broader spectrum known as the autistic spectrum which can include "high functioning autism". Hence greater the numbers.
But that does not explain the large increase. Doctors who have decades of practice are personally seeing more and more cases come in through their doors. Whereas in the past, their encounter with autistic is quite rare.
This I believe is due to environmental toxin. Areas near industrial plants have greater incident of autism. Some believe that autism is an auto-immune related disease. And auto-immune diseases are on the rise and environmental toxin is suspected. All the foreign elements in our environment such as pesticides, vaccines, mercury, is confusing the immune system and/or not allowing it to develop properly. Hence, the immune system incorrectly target self cells.