I think the market their games have currently appealed to aren't big enough.
Usually, 5-7 million selling games reach what TheSource calls "critical mass", and start hitting system seller territory.
Leatherhat on July 6th, 2012 3pm. Vita sales:"3 mil for COD 2 mil for AC. Maybe more. " thehusbo on July 6th, 2012 5pm. Vita sales:"5 mil for COD 2.2 mil for AC."