Why didn't Canon use the 1Dx sensor for 6D

the difference between an 18mm FF and a 20mp FF senor is negligible, so why spend extra on a newer sensor for the lower end FF body....this has been playing on my mind lately.  I'm impressed with the image samples I've seen from the 1Dx up to now so will this be a better sensor in terms of noise DR?  Or is Canon up to it's old tricks of protecting the Pro bodies and not letting it's technology trickle down to the lower end as they used to do with the AF systems

any thoughts

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