bbPress.org

It could work like that I think if there were no website (like WordPress) in the www root. Otherwise, how does the server know it’s a bbPress page vs. a WordPress page?

If you are running a forum only on a domain, then it would be silly to have it in any sort of directory at all. Otherwise, if you have a website in the root, you have to put bbPress into some sort of directory to prevent confusion.