content: Because it's the biggest!  In all seriousness though, that might be pretty much it.  It's the same reason 6TB hard drives are so much more expensive than 4TB or 5TB: 6TB is the highest you can get, so there's that extra appeal that keeps that price as the outlier.  8TBs are becoming more available, so 6TB will drive down and the cycle continues.  That's one of the reasons I look most closely at the second-best I can possibly get: that's often where the best value is.