"done with the story" is just another way of saying they are tired of God of War imho. Plus every game released has big games to compete against. There are multiple factors. I simply think people are tired of the franchise.
Look at PSP. The first GoW game sold almost or over 4 million IIRC. The 2nd game on the system in the franchise sold less than half that. And that game came out after GoW3. So maybe after the story concluded people just decided they'd had enough.







