I may be late to this discussion, but I think what most users including myself have issues with the 6D is that as a consumer you feel Canon is NOT giving you "great value" for your money when they take the AF system back to one center cross-type, while Nikon doesn't seem to cripple their bodies in the same way. It was biggest the complaint about the 5D Mark II, and Canon addressed it well with the Mark III, only to revert back to it with the 6D. My memory is not that short term.Also late to the topic but this echoes my thoughts too.
Canon not happy with 6D sales? More like their deliberate gimping of the 6D has come back to bite them (FWIW the 6Ds seem to still be quite highly recommended, including excuses of "no body needs more than 3fps ).
So all I can really say is the same thing as before. If they made them all cross-type at say 5fps, it probably would've sold a lot more without costing 5D3 sales.
I think this is the problem. Canon made the same mistake with the 6D as they did with the EOS M, a seriously underwhelming focus system. The cameras work well for portrait, landscape and architecture. For any other use, the photographer does the work. I say this as someone who owns both and likes both, but would have greatly enjoyed a focus system commensurate with the quality of the photos the cameras are able to produce.