Blizzard wanted to sell copies of the Diablo III expansion.
They weighed that desire for profit against the revenue they were generating from the real money auction house of a game released almost a year and a half ago in a heavily saturated games market (i.e. people tend to play a game for a short while then move on to the next big thing).
Looks like the potential revenue from expansion sales won out.
I'm not saying it's the only factor (I'm sure the positive reception of the console version of the game played a role as well), but it certainly had to influence the decision.
And speaking of the console version: Diablo III for PlayStation 3 and Xbox 360 features gamepad support, offline play, and no RMAH...which makes it a game I would actually consider buying if not for the occult content (which is a personal decision and not one I would try to impose on others) and $60 price point (though patience and Amazon sales could resolve that problem).
But for now, I'll stick with Dungeon Siege 3. DS3 was 5 bucks on a Steam sale when I bought it, it features gamepad support and local co-op, and I've really been enjoying playing it with my wife. It's not on par with Soma Bringer (then again, what is?), but it's been a great game to play together (though the camera is occasionally obnoxious).