User blog comment:The Inceptionist/A Multiplayer DLC Idea/@comment-1780148-20120730230947/@comment-5105853-20120730233551

The Collectors, or at least most of them, were wiped out when Shepard destroyed/disabled the Collector Base. The Multiplayer is considered canon and is tied in with the plot of the single player storyline. The entire point of Mass Effect 2 was to wipe out the Collectors, so the only way to involve the collectors in Mass Effect 3 would be to reveal that there are MORE Collector bases, or to ignore the whole suicide mission altogether. Putting the Collectors in game would either contradict the established plot, or make the entire 30+ hours spent gathering a loyal squad and potentially losing them in Mass Effect 2 a complete waste of time. (Then again, a lot of fans consider all this to be a waste of time anyway due to the endings, but still...)