The answer is far more simple than most people seem to think. To create a game that sells over 5 million you must accomplish one of two very difficult feats. You either make a game with a ridiculously broad base of consumer.
Sony franchises do not typically go for universal appeal. The two that I can think of that have tried recently would be Ratchet and Clank, or Little Big Planet. Neither were quite broad enough for various reasons that I don't care to discuss.
The best in the genre, or market is a problem of competition. They have extremely good third party support. Consequently many games they have fail to become the best experience in the eyes of the consumer. This splits some sales between the two games, with the percieved better one getting the lion's share.
All of the above statements are meant to reflect the situation at this very moment. I am not saying none of the games currently released won't, or can't make it. Simply that they have not yet. Any and all analysis and opinions offered could be very different in the future.