Alright, may have been covered in some of the earlier posts, but it probably has to do with render times and upload times. As many people have said, it takes much less time to export and upload something at 720p, so it is very tantalizing for someone who makes a dozen videos a week.
Also people who are watching youtube are typically watching on a computer so you don't really need to go up to 1080p, since the pixel ratio isn't gonna look to different on a smaller screen. Another factor is, perhaps they are shooting the original source material in 720p? All of these are factors to consider.