I'm too lazy to do a thesis, and I'm not familiar with English, so...
Doom9 is uncomfortable for newbies, it do not has modern features like
Autoemaling notifications
Mobile view
New posts/threads
Fast signing in using [s]icq/jabber[/s] gmail/facebook
Comfortable wyc editor
Limited width (it is just not very comfortable to read very long lines)
So you cant just chat and have fun
It uses very old concepts, from side header
To other places
It does not see relevant (recent post fixes that) for user,
If you go to this place you will see a lot of uninteresting and not actual category, it seems it focused on DVD, but its not actual
Usually inactive categories are hided until you do not want to see them or merged with others.
With old concepts it attractive only for few users, mostly geeks. So projects do not get feedback from users, projects are unattractive to them and for creators too. Even if some of them are popular, they can look much better and have better usability. For example, MadVR looks like WinXP and its not friendly for 'randomuser', it has not traditional installer/simple view/
Updating/upgrating forum to modern forum should makes it more attractive. It could be an alpha release in first time with moved database in different place at least for testing
xenforo is wow, like nfeteam.org/forum . it can looks dangerous to change all for people who get used to it, but its still possilble even in same vbulletin but more modern version.
https://www.vbulletin.com/forum/
BTW you just cant see existed topics in most cases, cause default policy hides 'inactive' threads and you cant switch them easily and for ever. May be it was good in 2009 to reduce forum load and stimulate activity, but now it means that >95% of forum threads are dead.