We don't know for certain why Dragon Age II no longer appears from Steam, but what is certain is that it is no longer possible to purchase the game from the service.
However, I can hazard a few educated guesses as to why Valve—and not Electronic Arts—removed the RPG from Steam. The new DLC for Dragon Age II, Legacy, uses the game's built in system to get players to purchase it instead of using Steam to do it. There is no question that this method of DLC acquisition comes in violation of Steam's terms and conditions, which require that DLCs be available through Steam, and not through the developer's own system. It's highly doubtful that EA pulled the title from Steam.
That being said, neither Valve nor EA have yet to comment on the game's sudden disappearance but it wouldn't be surprising if EA spoke up first, much as they did the last time this happened with Crysis 2.
It doesn't help matters that EA's Origin service comes in direct competition with Steam, a development which surely ruffled a few feathers over at Valve's Bellevue headquarters.
The question now is whether it's okay for Valve to retain such exclusivity to games and their DLCs in its terms and conditions. There's no question that Steam belongs to Valve and they reserve every right to do it, but we must ask ourselves whether it's a good thing for the industry as a whole for Steam to monopolize digital distribution.
This goes a step further than the complaints against retailer monopolization of pre-order exclusives.
