The Announcements forum is an official announcement board. If everyone could post topics in it, there would be a large number of fake announcements.
The Scratch Helper Groups forum, can only be posted in by the Scratch Team and the Scratch Mentors. This is because it's run by them, and they do not want Scratchers posting unofficial topics in that forum.
Archived forums are kept for historical purposes. It would be prone to vandalism and spam if posting in these topics was allowed.