Well its expected.

The game is story driven and people usually beat the story within a month, Dungeons don't offer anything but outdated gear not worth making into Materia or even selling, mytholigy and phil is all you need but once that is done nobody cares for the dungeons.

No reason to do them anymore once you unlock CT, No reason to do roulette because you can buy peacock ore and other items to sell on market from spamming CT for phil after you max everything get your gear peace then you are done for 5-6 days nothing else to do, some people just wanna play 1 job because their favorite job is not in the game so they go back to FFXI.