There is a reason for this - apart from the 6d sensor image quality the 5d3 is superior in every way.
If you happen to shoot center point only, shoot with slower lenses and don't track, go with the 6d. Remember that even *if* Canon releases a newer 5d4/6d2 anytime soon, it'll be a lot more expensive than the current models.
I wouldn't say 5D3 is superior in every way, the 6D center point focuses better in low light and it also adds Wifi+GPS if that's something you want. Also, I'm not sure that the 5D4/6D2 will be much
more expensive than the current models; Canon's pricing strategy lately seems a bit more reasonable.