I think because some class of people wonder if a $5 app would better perform. I think it's hard for 53 to succeed that way however, because you have a finite number of users.
By making features an in-app purchase, they effectively monetize users going from version 1 to version 2, and so on. If you used the up-front payment model, new versions of the app would have to be a different app to continually monetize it, which would have a number of it's own problems.
