Altair was too cool for demos, and so is Ezio.
Liked Assassin’s Creed? Well if you did, then I’m guessing you are buying the sequel, Assassin’s Creed II. But on the other hand, if you didn’t like it, or if you didn’t play the original at all; well, you aren’t getting a demo to see how the sequel improves upon the original. You’ll have to buy the real deal.
Still, this is a more logical decision than anything, as the primary audience for Assassin’s Creed II would be fans of the original game. Why risk warding them with a demo when you can get them to buy a copy any way, all for the precious sales. The precious salessssssssssssssssssssssss.