I gotta tell you, I think some of these issue are some sort of performance anxiety for these developers. It sounds a little crazy, but if you consider the hours and months, sometimes years that they put into these games. I think it almost kills them when the game flops. So, when they find one that works, it takes a lot for them to put new twists into them because they know they will hear the comparisons.
Think about the Criterion developers and what they had to go through with Burnout Paradise. They were hated by many and adored by the other half. They are going through it now with NFS.