That could be due to how much easier and similiar majority of the FPS games are nowdays. Most of them are very easy to learn and the combat is more about who notices the enemy first rather than who is better marksman. I guess all of this "veterans go away" would be gone if majority of the FPS games had a proper learning curve.
It could be also due to the increased amount of forum users. A lot more people visit and post in forums, resulting in more of the "soft" players (Not trying to offend) showing in the forums.