To cut a long story short, they're time consuming to create and manage, and in at least 2 cases, they were created but sat completely dormant with no posts at all. We would really rather not have forums sitting there that aren't being used, especially given that they would soon start to add a noticeable amount of bulk to the index page.
If there are any active (and sizable) gangs expecting a reasonable number of posts per month, please feel free to PM me and I will happily create a gang board, though.