Trey Parker and Matt Stone are making baaaaaaaank with their broadway show, The Book of Mormon. It's constantly sold out and tickets aren't cheap. It's up for a bunch of Tony awards, and if they continue to gain success with this, I can see them producing/creating/writing more musicals. Maybe they are just trying to focus on one project, and move on.
Not that they should, because South Park has always been entertaining at the very least, but I think they're pretty stoked on their new ventures. They've wanted to do musicals after the success of their movies, and since The Book of Mormon has taken off, I can see them staying in that direction.