Now, you see, you can't quite raise activity levels by removing the tracker. There was still a pretty considerable number of players, by Discovery standards, still logging in just to farm activity.
Onto the reason why activity is at a low point, well... It's awfully simple, to be honest. People get bored of the repetitive things done here and I'll say this myself, as I was one of the most active player a few months ago, that once life catches up with other matters, you stop logging as much. As soon as that happens and you've got limited time to log on, the game becomes frustratingly stupid and boring, to the point where you lose any interest to open it up.
It's a cycle that has repeated for many players and for many years. Unfortunately, from my own perspective, one cannot do much to save this but rather just try his/her best to enjoy as much as possible. In the end, it's a game.