What happened to Army of Two?

Army of Two enjoyed 3 entries from 2008 to 2013, but the series has been dormant since the release of Army of Two: The Devil's Cartel. What's happened to the co-op series Army of Two?

Army of Two tried to be a series that emphasized cooperative teamwork with new mechanics like the Aggro system. Your teammate takes Aggro while you can sneak by and vice versa. Among other tricks, Army of Two was unique for it's forward approach to co-op gameplay. So why did the series vanish?

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