Because it SOUNDS like a good idea, but is really little more than a gimmick. Once you've gotten over the "newness" of it, the only time you're likely to use it if you aren't sending movies back in when you're done with them. And because of how easy it is to mail them back (walk them out to your mailbox), you'll be more likely to do that then to hold on to them until you can get to the store (if you even have one close to you).
This is probably only going to seem like a good idea to someone who likes to look at the boxes to help them decide, and these folks aren't likely renting enough movies to have too much of an impact one way or the other.
Which brings up a good question, what kind of new subs are BBI getting? Frequent movie watchers or ones who will cancel their memberships once they realize they aren't watching enough movies to make the monthly fees worth their while? I would appreciate a link if anyone's seen any mention of this kind of analysis.