Hmm, I think you are a bit wrong about it. Bitcointalk is the official forum for bitcoin itself, although the discussion on bitcointalk forum covers all areas of crypto space and lots of new projects launched here it does not mean bitcointalk is their official forum, they can create their own forum later.

The reason why lots of new projects making a debut here because bitcointalk is the most active crypto forum (based on forumrankings.net), so there will be a chance they can embrace more people to follow the project by post their ANN here.