Originally Posted by kingmatte
A couple of reasons. First, they've already taken one Thor villain and made him an Avengers villain. Second, all signs point to Thanos having some involvement in the first Avengers movie, so to bring him into Thor 2 would cheapen his appearance in Avengers 2 (IMO).
I agree. There are certain rules for storytelling and build ups. IF Thanos is somehow featured and foreshadowed in Avengers, then we won't see him as a main villain until Avengers 2. He MIGHT turn into the Nick Fury of Phase II, though. Popping up now and then after the credits.