Easy -- it would infuriate the 5d3 adopters. Plain and simple. I wasn't aware the d600 had the same sensor as the d4/d800/d3x. Even at 24Mp, it's well known that the d600 sensor is a newer (and better) sensor than the d3x.
Those of you who need all that the 5d2 or 5d3 offers will buy it. Besides, the 5d2 should ultimately still prove to be the better overall dslr than the 6d. I'm Nikon shooter now (having owned the 5d2) but miss much of Canon's system -- like the 35L, 100mm macro, etc. Canon will convince me to buy into their system again as well....it just may be years down the road. I know this is pointless -- but I wanted to state that their TELEPHOTO lens selection is simply unparalleled. This is why they OWN the sports photography market. As far as landscapes....they are still very good. It's a tough time for Canon right now, but i DO HOPE they pull out of this "slump", if you will.
Except for 70-200 f/4, both Canon and Nikon have very similar telephoto lenses. Nikon even has 200-400 that Canon does not have, and I have seen it many times in the fields. Why would you say Canon own the sport photography market?
Because Canon's 300 f/2.8L, 400 f/2.8L, etc. are way, way better than Nikon lenses. Overall, Canon has better sports lenses, and then of course there is the 1DX which is superior to any Nikon camera in existance.
...but they're not?? nikon has a 300/2.8, 400/2.8, 500/4, 600/4 just like canon, and also have had a 200-400 for ages, whereas canon's is still nowhere to be seen. Everyone's superteles are very very good performers and nobody would be disappointed with either brand here.
The perception that Canon destroys Nikon for sports/wildlife stems from the early days of EOS, when Canon's quick jump to AF caught Nikon off guard. Since then both brands have more or less equalized and both perform at very high levels.
Also 1DX better than any nikon camera? Sure, if you don't count the D4 or D3S. Again, both brands offer superb performance once you get up to spending $6k on a body and $5-10k per lens.