Since Canon knows how to build the 200-400mm lens now, many of the production risks go away. This means they know the cost to produce such a lens with good certainty.
Size wide, it would be only slightly larger than the 200-400, and price wise only slightly more. Since Canon knows the actual sales figures for the 200-400, they also can estimate sales for this lens. With money being tight, its a matter of putting the money they have to invest into products that return the most profit. There is also the pride in having the best telephoto out there in a focal length range, and the new Tamron certainly must have them worrying.
I hope a miracle happens and we see one for under $10K.