5D3 vs 6D

Another way to look at the 6D would be as an increase of 2MP over the 1DX.

Regardless, at this point we have no way of knowing whether there’s an engineering reason for 20MP or a marketing one.

By the way, speed isn’t only about FPS.  It’s possible that Canon went with 20MP to allow more shots before filling the buffer or to allow more time for AF calculations between shots.  Or it could be related to video capture.