Any forum relies on its members to generate content. The problem is that as more experienced and long standing members have left, either because they have stopped playing the game, or have too many commitments to post regularly on a forum, younger and/or newer players have not come forward to take their place. During the first decade of the 2000s, so many new members would join that the disappearance of others was rarely discussed, but as more and more people now use Facebook, Twitter, Reddit, and other similar platforms, the loss of experienced members has begun to bite. Throw in a major server problem which adversely affected activity and membership a few years ago and this is why there are far fewer contributions than there used to be.
Other forums have the same or similar problems incidentally. It is not specific to 40K Online. The only way to reverse it is for more new members to join, but prising them away from all the other forms of social media out there is, seemingly, mission impossible at the moment. Again though, we rely on our members to spread the word about how marvellous this place is to hopefully draw in fresh blood