Here's a few reasons I can think off the top of my head.
There's pretty much zero reason to grind rep, therefore an even smaller reason to afk, which leads to less players (What Iron said). Why would i grind rep when i can just buy it for 10 bucks and get 10k? Along with rep problems, there is no reason to level up your gang, after level 23 or so the prices for rep are fucking insane, for an extremely small reward. Even if you do become top gang, someone can just buy rep to level their own gang above your own.
There's been little to no events to help maintain population, i.e. there is a lack of re-playability with lack of fun and new leads. X-mas in July was a good event to help keep people playing in July. We don't need an event every week, but to have one once a month would be good to keep people playing on the server.
Like SPlink said, there was a drastic change of the way JB was played while a bunch of (i know their kids but shit) annoying kids who aren't even old enough to have CS trying to lead while micspamming. If i have to mute someone more than 20 times in an hour for mic-spam you shouldn't be talking, which makes me as a mod not want to moderate, let alone play the server.
I've heard absolutely nothing from staff as to plans to help revive JB, or really and CS gamemode and everyone has lost the will to play again. I want just as much as everyone else to join a 48/48 JB again, but if as a mod team we aren't communicating let alone playing, we all shouldn't be mods+.
Of course i don't know really whats happening with leadership, but it seems like practically everyone in staff has IRL stuff keeping them occupied.
But who knows, cus I sure as hell don't