Why is it those with aspergers can seem so intensely focused on astrophysics but really nothing else?

Seems like psychology would be a really stimulating field of study, and could benefit from their intense focus. But why is it something like astrophysics can grab their attention but the field mention previously wouldn't? And I know not all are focused just on that, but a great deal seem more focused on the very difficult fields that require massive amounts of focus and use of the mind for extended hours most would be exhausted mentally.