COLOR SPACE
Excerpt from the Canon Boot Camp Manual
Another problem the Canon engineers faced was whether to have the 5D record movies in 4:4:4, 4:2:2, or 4:2:0 color space. Actually, since they weren’t planning to redesign the whole camera, 4:4:4 files would have been too big for the 5D.
Again, the engineers worked backward from what the eye could see. AND the size of the screen the audience would be looking at, i.e. a High Def TV screen.
So, they set a realistic goal, rather than a goal of ultimate perfection that would make a camera too big for the Associated Press, and make its output so data heavy you’d have to buy a whole new computer to edit it. They took a practical approach to achieve AP’s goal.
So, (we think) they decided on 4:2:0. Why? Because it saved a ton of data storage and most of us can’t tell a difference once 4:2:0 is upconverted to 4:2:2 Pro Res.
Below is a link to a video explaining the concept of color space and how it works.