This is a post by someone without the first understanding about digital cameras - cue the smiting!
The 1Dx is not about high MP counts, but it's still an amazing acheivement, and does what it's supposed to in an amazing technological tour de force. It's all about high fps 14 to be precise, it's a camera for action photography, up the MP count and the frame rate would fall, which is not what the market wants.
As for Nikons 12MP sensor, many users love it, end users only want web or low res print and even for bill boards 12MP is enough. PP is faster as is file transfer, there's a whole host of advantages to lower MP counts. Then perhaps you might like to check out just how much as a percentage larger the 21MP canon senosr actually is. The funny thing though is the way that Nikon users are looking forward to a 36MP camera extolling the virtues of high MP count sensors while forgetting the arguements they used to defend the 12MP they currently have.