Also the reason BTT survived?
I think BTT is just special and maybe some things like signature campaign has been a key to keeping here going.
The reason forums got outdated could be because of lack of the modern day UI/UX amongst other things.
The forum is outdated because only one person is responsible to update files and database. Theymos already made many changes in the forum core, I am no sure if all these are documented. If it is not documented then it will be harder to check all the updates and restore to any other theme [not just SMF but the new forum software if we ever move].
The different kind of advertising idea helps users to stay active in the community, everyone likes to get something in return of doing something that they always do.