Its either that jj2 lost its popularity bc ppl dont like it anymore, or other responsibilities became more important to ppl that they had to stop playing in cd. jj2 is not fully inactive, as shown on gip, ladder is substantially inactive and this 2v2 tournament is not quite continuing yet. so its hard to predict when or if the ladder scene will regain activity, or the players who were making ladder popular/active r rly done with the game.
I think it's more likely down to the fact that JJ2 has just barely enough people who play CTF to have a decent competitive clan scene at the moment. The ladder has gotten rather stale as most of the time, it's the same few players fighting the same few other players in the same few maps with the same format. I think more variety would make the ladder interesting again, at least in my opinion.