Because this board is active. Same reason 360 people all go to the PS3 boards pre-release for fighting games.
This is why I'm here. I check the PS3 boards too, but it's much less active. Once the game has been out for a little bit, the PS3 board will be active enough that I won't need to come here anymore. It's the same cycle every year for CoD on the forums.
I own both consoles but use my PS3 for the majority of my online gaming. And yeah, the 360 boards are almost always more active, (especially for FPS). I also owned a 360 first so I am used to posting on these boards.
TC if you look on the PS3 top board you see a different set of game while 2 cod games stay on top for xbox so they come to this board and PS3 gamers are having a blast with the long list of exclusive to choose from.