Because it prevents advertising, But youtube videos are more than allowed. Just put your youtube links between either one of these codes and it will work fine. Or you can use the code button that looks like a tv on the top right of were you type to post and pic which video hosting site you would like to use.
Here is the youtube codes,
Code:
[yt][/yt]
or
[youtube][/youtube]
If you have any further questions on this please make another thread.