The reason new players are leaving is because it's so hard to "git gud" in this game. You can say Battlefront is for brain dead, but it's new-player friendly. MB2 isn't. MB2 favours the elitist players who played for many years. Everything in this game is complicated and complex. Especially the sabering system. There isn't even a proper in-game tutorial. There are only written tutorials and the library, and people don't come here to read pages long just to learn how to play the game, not to talk about any more advanced stuff (I, for instance, still don't know what ACM is despite playing for many years). You literally have to be an autist and to break your mouse to swingblock between consecutive swings and perfect block, including that you've played for several years at least to perfect it. What can a new player do? He enters the game and gets raped by an elitist who calls him a "noob". The end.
Another reason why players in general are leaving is because this game hasn't been fundamentally changed since 2008. Yes, there have been new maps, new models, certain tweaks and small additions to classes, etc., but the gameplay and classes are fundamentally the same and no new classes were added. I've seen the videos from 2008 and was negatively surprised to see that the game is fundamentally the same right now as back then. MB2 is stuck in a constant loop of fixing what's broken and what's not broken, that it just fails to deliver any significant new content. Every game gets boring if this is the case.
And finally, probably the most dominant reason MB2 is dying is because this game is 14 years old. JKA no longer gets any attention anywhere after 14 years. JKA is dying and MB2 is dying with it. What do you think, that people would try JKA today while more up to date games with impossible graphics, gameplays and engines than ever? No. They don't even know about it, and much less about MB2. Now, I started playing video games about the time JKA was released (but not JKA at that time yet), and still the best graphics for me are the graphics of that time. New graphics are just too detailed for me and overwhelming and everything. New games are more about the graphics than gameplay. But still, the graphics, for example, from Dark Forces and older 2D games, I just don't like and I would never play those games, while there are people who played them back then that do love those graphics. What do you think, that people who are used to new graphics would find JKA alluring? No. It just looks bad to them, just like Dark Forces looks bad to me.