I wonder why wp don't use phpbb
Because, that has been discussed over and over again, and it is simply becoming annoying. The WordPress developers have chosen to use BBPress over phpBB because they also develop BBPress. BBPress is a work-in-progress. This is the testing ground. And, it gets the job done. As far as I am concerned, the only things that BBPress lacks are all of the bells and whistles of other forums, like avatars and polls.
BBPress is a bare-bones forum system. It supports registered users, categorization, tags, topic history, resolved/un-resolved status, and a search engine (when properly implemented). A forum needs nothing more.
I try to search "html code" on this forum, turn out no results.
Matt wanted to release a new wordpress.org along with WordPress v2.0, and many individuals wanted WordPress v2.0 to be released ASAP regardless of any consequences. As a result, some things here are horribly broken. The search feature is one of those things. For now you can either use the old search engine or search this site via Google by adding
site:wordpress.org before your search terms.