Because Rovio doesn't want to make them?

Well the real reason (I'm guessing based on the same reasoning other companies aren't making apps for Maemo is). There's not enough users to justify the cost.

Think about it, how many N900s out there. We know the minimum is around 100k based on what Nokia's released. Now compare that to how many Android or iPhones are out there. If you were a company/developer interested in profit, would you spend time and money developing a game that could only sell to that 100k audience? And remember, not everyone may buy it.

Sidenote: This is why Meego and Symbian 3's success with QT compatibility is needed if Meego/Symbian 3/and N900s running Maemo ever want commercial apps enjoyed by Android and the iPhone.

The reason why you see Angry Birds from Rovio, is because it's already created for another platform. The iPhone. They just have the port the code over, and it's not as hard as writing it from scratch.

Now backto Bounce. I believe Nokia paid Rovio to make the tech demo for the N900. But unless Nokia pays Rovio to finish it, you won't see anything else of it since it's not just a port of the game on another platform.