The problem is:
Lazy developers.
Why bother to make up your own concept, do something unique and draw in people with a cool art style, new weaponry, unique locations, fun multiplayer/co-op options, or interesting gameplay when you can merely look and see what games moved a million copies, make a similar game, and push it out banking that you at least capture a percentage of that consumer base that keeps eating up FPS or 'historical war' shooters.
Its the same reason you see a deluge of mini games on the Wii, its the same reason you see hundreds of MMOs announced on the PC every month, its the same reason you get knockoff games that try to be like the Marios, Zeldas, Castlevanias, Halos, GTAs, Gran Turismos, and the like. Why pay a team of guys to make up a new concept when you can pay one guy to study market trends, look and see what 'cool features' those popular games have, and then incorporate them into yours?
Edit- Not saying shooters are fun, but what if you like a broad range of games? Chances are, your system gets stuck with this stigma that only kiddie games are coming to it. Or only shooters. Or that noone owns your system and it doesn't matter.
