Really? How do you come to that conclusion? On a chip that is neither natively 5:4, 4:3 or 16:9 aspect into the bargain?
I'll let another user answer that for youa slight increase to 22mp is intended for EOS-Movie, horizontal pixels 5760 is exactly 3 times of 1920, this indicates that 5D Mark III will do a full-sensor readout and 3x3 pixel binning to produce clean, sharp and almost-no-moire HD video.
Now, that's hypothetical for the reasoning, though it makes sense to make a sensor that can be used among many cameras (including the C cameras) that works well with video capture. But, they might also have found 22mp to be compromise between low-light and resolution that they liked best.
That moire is basically non-existent in video would lead me to believe that the sensor size isn't accidentally a solution for that, but, I wouldn't say its the only reason. Canon settled on 21mp for the 5dII with video as an after-thought; it might just be the magic number for them
Are there any components where compromises have to be made which will detrimentally impact the design for a still camera? - for example 22mp is a video size, perhaps it might have been as cheap and easy to make a 28mp sensor which was optimised for still image
It's an interesting question. I'd have to imagine that there was a slight compromise there...though, a true video person would be even happier with 4x4 binning, which would mean a sensor in the 35mp range. Canon have made sensors MUCH larger than that, so, they are clearly capable of such a thing. I imagine they tested it and decided that for the target market (wedding, event, indie movie set), low-light handling was more important than that added resolution, and their sensors only allowed them to go so far.
So, sure, it could have been 28mp, but, do you really need that extra resolution if it cost camera a stop of light, or didn't solve issues from the older 5dII (moire, etc)