There are a few reasons for that in my opinion. This first one might be debatable depending on opinions but I think that years ago gamers weren't as used to games holding your hand and being as polished as today (I am aware that there are incredibly successful exceptions to "polish" such as PUBG still). So more players were fine with finding out how to do things on their own without any real proper tutorial that tells you to press WASD and M1. A lot of games nowadays are pretty fast to get into and are very polished (for example Fortnite, Apex Legends or even GTA Online, if you want to compare with a somewhat similar game). So when new players get into APB they see features similar to GTA where you steal cars and shoot civilians but they often don't know about how missions work and a lot of them don't read/do the tutorial. They just wanna hop in and game which doesn't work with APB at all right now. This is likely one of the reasons BR games are so popular these days, because you can get into a match very quickly. You open the game and you hit play and you're instantly in a match already, even on mobile devices. With APB you can't even join the right district if you don't already know a thing or two about the dysfunctional matchmaking system.
Secondly, it is in the nature of online games to have some amount of players when they first come out and there is a level of hype and if it's overblown like some newer titles nowadays, a lot of people flock to them. But then as time goes by and the hype dies out, more and more of those players leave the game to go play something else and only the players who have continuous interest in the game will remain committed for the next years. In APB you can be competitive or you can enjoy the customization or you might enjoy the community (lol) or something else. There is a reason for each of the remaining players to remain because we've already climbed the long ladder of getting settled into the game and learned our way around it. The game was more lively when most of us joined it so a lot of us feel nostalgic about it too. When new players join now they see a broken, dead mess that stutters and caps out at 80fps on a 2019 PC, they get utterly devastated by veterans left and right so they leave, wondering how anyone can play this pile of trash. We're simply so ingrained into APB that to us the game isn't half as bad as it looks to new players coming in now.
And APB simply is just extremely unfriendly towards new players and it always has been this way. The game, as is, isn't for everyone anyway and the amount of time and effort you need to put in to get to the level of understanding that most remaining old players are is just way too much to ask for of players who probably just want to play GTA anyway because that's what APB was advertised as (but it actually isn't GTA at all). Like others have said on the forums, it is being advertised to the wrong crowd of gamers. Then you have the broken matchmaking which forces you to deal with veterans from 2010 who utterly destroy you, the game gives you no real tutorial and it doesn't tell you what districts to join or a whole lot about unlocking mods or weapons or anything at all. And if you do decide to stick to it you are probably going to end up thinking that a large majority of the players are cheating because you stand no chance against them. On top of that the game runs like complete wack if you don't use configs and you don't have a top of the line PC from 10 years AFTER the game had even released. Who in their right mind would want to get into this game in 2020? That is the question we and LO should be asking ourselves.
I'm sure that most of us have at some point attempted to introduce our friends to the game but almost no one stuck around because they just saw better options out there and they will keep seeing better options unless APB becomes proper up to date and new player friendly (which does NOT mean making it casual, those are two vastly different things). It needs to be a lot more accessible than it's ever been, especially if it wants to attract any type of competitive crowd. I believe that APB is still the best third person shooter out there and is unique, it has competitive potential and working on that potential is the only way to get this game off the ground in any capacity at all.
My 2 cents.