In the case of The Simpsons, option 1 is the reason....

Generally if a game is not for rent it is because:

1. The game is older and out of print. GameFly has sold all available copies or they have become damaged or lost.

2. The game requires a special controller or piece of hardware.

3. The game is not released yet, and may become available in the future.

In all three instances, GameFly keeps those titles on the Web site so you can still access the content that is available, even if the title is non-rentable and cannot be added to your GameQ.