There's a period of a few years after a big game where every publisher under the sun has to put out their own take on it. You've seen it. Grand Theft Auto breeds everything from True Crime and Saints Row; Modern Warfare's success results in Bad Company, new Medal of Honor, and more. We're right on the cusp of about a thousand Farmville clones, too.
Obviously, some of these games are good. Some argued that Saints Row 2 was more what GTA fans were looking for than GTA4. Battlefield: Bad Company 2 has won over its fair share as well. Most of the games are just also-rans, very rarely are there actual contenders. Just like anything else that becomes popular suddenly, there's a temporary deluge of the given game type before settling into its place in the gaming genre pantheon.