The original Assassin's Creed was supposed to have full drop-in-drop-out co-op support throughout its campaign, but then Desmond happened. OXM UK spoke with several members of the game's development staff, who said plans changed once they found out the game wasn't straight-up historical fiction.
Assassin's Creed's engine was similarly inhospitable to the concept, and eventually co-op was dropped. Multiplayer wouldn't figure back into the series until Assassin's Creed: Brotherhood debuted its unique competitive stealth modes, presented as a training simulation by the villainous Abstergo corporation.
"There was no way to reconcile having multiplayer or co-op in an ancestor's memories. Your ancestor lived his life in a certain way, so assuming you had branching storylines, it creates a paradox. It didn't fit."
Some might say the opposite thing happened with Dead Space 3, eh?
