I wouldn't necessarily say the 1DX raw is "better". It is in some ways and not so in others. however to answer your question, consider the two sensors 23MP and 18MP both full frame. The pixels in the 18MP sensor are able to capture more photons per pixel than on a 23MP sensor. In order to produce the same exposure between the two, the 23MP sensor must be amplified internally to compensate for fewer photons. This is why the shadows sometimes suffer on the 5D3. There isnt enough photons down under to pull out shadow detail. On the high end, the 5D3 loses as well because the amplification is to high for the bright pixels.
So in a nut shell, the 5D3 has to undergo some internal amplification of the photons detected to achieve the same exposure. It's not good or bad, it's just what has to be done when you spread out the photons onto a higher pixel density sensor.
So back to one of my previous statements in that you can get the 5D3 to perform just like the 1DX and in some cases better. Just have to realize that the lower 3 stops of the DR (both cameras) comprise of less than HALF of the available DR..and that means that the 2 stops on the high end make up more than half. If you bias your exposure toward those two stops on the high end you can pull down your shadows and get similar results as the 1DX..at least as far as shadow detail is concerned. Not many people are savy enough to know how to get 5 stops of DR in their photographs and that's why there is HDR process.
I personally like the 5D3 raw files better than the 1DX, not from a DR perspective but from a noise perspective. It's just smaller, more random and easier to deal with than the 1DX. Perhaps the 1DX is better at super high ISO but I still find the noise to be bothersome compared to the 5D3. That's me and others will differ in their opinion. I wont be buying a 1DX but I'm waiting to see what the next generation will be like. Hopefully an improved photon amplification system and better signal to noise ratio.
I have heard, increasingly lately, that 1DX RAW files are better than 5D3. I have noticed myself, in other posts, that they tend to be able to be pushed farther. Why is this? Thanks for any insight.