I don't think they ever gave a reason. Personally I like how it has forced developers to come up with more creative product offerings than time limited demos. While convenient for developers, time limited demos suck as a user experience IMO. The whole "trial expired! buy now!" experience is very grating.
I am fairly certain the "trial expired" experience is exactly the reason. They're also trying to make a commodity out of software and having an app store half full of trial versions doesn't help them with that goal.
