When have you ever publicly seen a raw file from any Canon Prototype Camera? Never, thats right as it always has been and still is their policy to not release raw images captured with Prototype Cameras or Lenses. Once the production version is nailed down, you will start seeing raw images. Sure there are individuals out there who test prototype bodies and lenses for Canon but by contract, they are not permitted to share/release raw images.
In the case of the 1Dx, it is the finalization of the firmware which is holding up the release of a production camera I'm told. All the 1Dx's at PMA had their memory card slots sealed shut so to not allow attendees to try to capture images, this is normal. When they show new lenses at a show (that aren't production released yet), again they will not allow you to capture an image with them. They want to ensure that before anyone out their starts comparing raw images that they are doing so with a production release products. Prototypes can be buggy and do not meet the the final quality control standards of production release products and this is true in most all industries.
I will say that not only us end user are disappointed at the f/5.6 limitation, we have 99.9% of their tech/sales reps pulling for a fix to this issue.
As far as why you are seeing raw images from Nikon, I can only assume that they are further along with the D4 then Canon is with the 1Dx and that they have a production release version of their firmware. As you can tell, Canon chose to announce the 1Dx earlier in it's manufacturing stage than Nikon did with the D4. This is a marketing decision and it isn't right for me to judge them on this but it does make me salivate for a longer period than the guys on the dark side have to.