The fact that it's a forum by itself or not doesn't determine whether it'll be active or not. If people want to talk about the manga, they'll look for the manga section, and it's not too hard to skip it since its linked right there in the first forum of the forum category. I proposed that the Shipping Debates thread, basically the only shipping thing here, would be upgraded to it's own forum or just sub-forum. But of course, it's not going to work because it's only the same little group of people discussing in it with a few new people once in a while.
The Webmaster section was also it's own strong forum, but it was demoted to a sub forum of the Tech forum not too long ago because of activity. So see, there is no correlation between subject status, and activity, there is only a positive correlation between activity and status. And the reason there isn't activity much is because there aren't that many people that read the manga, because it's not anything big in America, Australia, or UK, the biggest countries were our population comes from.