There are two reasons really. The inconvenience of using a m/k when sitting on the couch or in a chair. Number two would be in the game's design. Due to the incredible ease of aiming with m/k, character speeds need to be increased. Look at the UT and Quake games, they just don't work on consoles. Halo on pc doesn't feel right either. I really have no problem at all using either input devices. I am quite efficient at both. It doesn't even matter when everyone is using the same thing.

SNES had one and it wasn't very successful

KB + Mouse are for desks. Consoles are for lounges + TVs. TBH actually the Xbox 360 would be better with KB + mouse support than the PS3 as it has a better scaler chip which supports monitor resolutions.

KB and Mouse are only good for realism FPS such as OPF Dragon Rising.

Controllers are best for fast FPS such as CoD and Halo.

What does scalable monitor resolutions have to do with KB and mouse support?

Probably because often the game is designed around the controller. Mouse is a lot more precise than a controller so PC FPS usually has a faster pace than console version. Most developers tries to keep everyone on equal ground as Key/mouse would give an unfair advantage over the console controller.

KB + mouse lives on a desk. Monitors live on desks too.

O, I see. I used to play Uncharted 1 on a computer monitor and the PS3 seemed to be able to display it..

I don't see why there can't be a firmware update to allow mapping of PS buttons to which ever key on the board, or mouse you want.

I'd try to play some games (fighters only) using the keyboard. Works wonders for Marvel Vs. Capcom 1. I need a 4 face button pad like the PS3 provides for MvC2, though... I play the two games completely different.

Now that I think about it.. Tekken 6 on a keyboard would be way too awesome.

the 360 outputs 5-6 LCD monitor resolutions the PS3 is often limited to 720P.