...it's probably for the benefit of Leopard and earlier users who must install it to get to the App Store before they can upgrade past it...
No Macs old enough to have shipped with versions of OS X earlier than 10.6 can be upgraded past 10.7, so that's a pretty specialized class of users: those who have been content thus far to stay with an OS version that's now more than six years old, but who will nonetheless be motivated to jump through the hoops required to upgrade to a three year old OS version they won't be able to advance beyond.
