I would say it is because their brain is less geared toward other subjects (such as social behavior, etc) and more geared toward a single subject.
In other words if 2% of the normal person's brain (and this is just an example) is geared toward social behavior, and 2% toward, say, art, then could it be that an autistic kid's brain is only 0.5% social behavior and compensated with 3.5% art?

This post was edited by Zangoose on Nov 6 2011 04:20pm

thats what 90% of the other comments are saying. Also, due to lack of social skills it gives them more time for themselves.. another increase in the firld of which they accel at

Autism is just a mutation of a chromosome. It's just a random a change in the development of the brain. This change can be beneficial in ways and detrimental in others. Some cases increase capacity in a particular area and said person is a savant, but it can also decrease capacity in other areas such as social skills.