User blog comment:Dragonzzilla/ME3 Multiplayer DLC I Ideas/@comment-5069762-20120501190136/@comment-4950455-20120501222702

The question is less whether they could do it and more why they would. The Collectors didn't really have a special advantage over more "modern" types of husk - the primary reason they stuck around seems to be so they could be used as the Reapers' "front men" when necessary. Now that it's come to the open invasion part of things, there's no real need to revive the Collectors specifically.

Not saying that I see any problem with adding Collectors to multiplayer, just that there are reasons why they wouldn't be brought back.