This warrants further explanation. EA wanted to sell DLC for Dragon-Age 2 via an in-game shop, bypassing the Steam store (and bypassing the 30% fee). Valve didn't agree to this.
According to EA, it was Valve that removed EA's games from the Steam store, not EA. That's the only publicly available information on the subject.
By the way, you said "big players", implying that there's another big player other than EA that no longer sells new release games through Steam. Can you name another one? AFAIK, EA is the only "big player" to ever have their games removed from the Steam store.