HTML5 is still very new way of displaying video on web. On an official YouTube blog we can read that the company has to use Flash technology in order to meet all the needs of a site like YouTube. HTML5 < video > tag does not provide yet a good enough experience for viewers as well as content creators.
An official statement for apiblog.youtube.com:
There’s been a lot of discussion lately about whether or not the HTML5 < video > tag is going to replace Flash Player for video distribution on the web. We’ve been excited about the HTML5 effort and < video > tag for quite a while now, and most YouTube videos can now be played via our HTML5 player. This work has shown us that, while the < video > tag is a big step forward for open standards, the Adobe Flash Platform will continue to play a critical role in video distribution.
It’s important to understand what a site like YouTube needs from the browser in order to provide a good experience for viewers as well as content creators. We need to do more than just point the browser at a video file like the image tag does – there’s a lot more to it than just retrieving and displaying a video. The < video > tag certainly addresses the basic requirements and is making good progress on meeting others, but the < video > tag does not currently meet all the needs of a site like YouTube.