The reason is because hackers can use HTML to seriously damage your site.  BBCode is also a lot simpler for non-coders to use on a general basis.  Really, BBCode should not be hard to learn at all for someone who is familiar with HTML. 

i personaly like perl and .asp most
also more can be done with perl .asp and php then in html

It isn't which is more popular or functional.

Javascript wouldn't be possible without HTML.
PHP isn't possible without HTML
Websites aren't possible without HTML

However, websites are REALLY boring with just HTML. So Javascript, PHP and a whole ton of other languages are used to enchance the HTML, not replace it.

BBCode is only used because it is easier to understand than HTML(in general), and it won't do any damage to anyones forums because it isn't actually a language. Your forum just takes the BBCode and turns it into HTML...and rejects all HTML that is tried to be put manually into a post.
Makes it so that nobody can mess with your forum by putting unwanted HTML in.