Because a 6D is a rebranded 5D2 with fluff performance wise. Why pay more for what essentially is a 5D2? Bad value.
Feel like I'm beating a dead horse here...I do not consider the ISO performance to be fluff. Why would I waste $1400 on a body that would only be used in outdoor ceremonies? A 6d is much more versatile than the 5d2 if you work in low light a lot.
The 5D2 was good in low-light from 3200-6400, and still is now. Not saying the 6D isn't better but once you downsize the NR-files to around 12mp print sizes, Its pretty close.
Feel free to check those out. Plus, 6D's AF perfomance is nearly identical to the 5d2