GT 5 still feels lke a demo, that's why. Haven't seen a Forza 2 Prologue tbh...
But it comes at a lower price which makes it reasonable. Allthough I think a price of 30 euros would have been better. It will probably take a long time before GT 5 releases so I think a lot of fans will buy the Prologue version anyway.
The Patapon forumtopic: http://www.vgchartz.com/forum/thread.php?id=14103