simple. they wanted to protect the 5DmkIII. if they sold you the same sensor, they wouldn't be able to do that. They also can't go high MP if they want to stay low cost so they had to cripple the 6D or risk nuking their own 5DIII.
imho that argument makes no real sense... the 7D has the same sensor as the 550D.
there is enough to differentiate the two models.
you don´t have to research and develop a new sensor for that.
But look at what happened with the 1Ds3 and the 5D2. The 5D2 pretty much killed the 1Ds3.
If the 6D had the same sensor as the 5D3 (or better) then you can bet your a** that the 6D would have taken a large number of sales away from the 5D3.
that´s correct but it was a special case imo.
a sensor is not the only thing that differentiates two camera models.
the 1Ds3 had the bad luck that the 5D MK2 was good enough at that moment for most.
but with the 11 point AF system and all the other shortcomings, the 5D MK3 sensor would not make the 6D SUCH a better deal.
and still the question remains... why not using the 5D MK2 sensor then?