It's right there in the forum rules.
Quote:Posting Permissions
To prevent users from incorrectly posting threads, some forums require users to have a certain number of posts before they can make new topics. The threads in these forums need to follow set templates, therefore only users that have been active in the community are granted permission to post new threads.
Currently active on the following forums:
Gamecube, Wii, Wii Misc, Triforce - Users must be in the "Above and Beyond" group (1000+ posts)
HD Texture Projects - Users must be in the "Advanced User" group (300+ posts)
Delfino Plaza - Users must be in the "Somewhat Advanced User" group (10+ posts)
The forums in Game Support already have threads for every game. That is why new users are not allowed to make new threads there. Please look up an existing thread and post there if you have questions.