It's 100% about having the right people, and twisting their arm a little bit.
Not only do they have to be GOOD at the game, they have to play it often. They don't have to be your friends, they don't even have to get along, but they do have to respect each other (no griefing or other such nonsense). Most importantly, they have to fucking play: there has to be people on the server all the time, and at least a couple times a week a handful all together. The best advice I can offer is that you talk to the people on your server and try to schedule once a week, for at least a few hours, where a bunch of people get on together. If one of them streams the content or is recording it for upload on a regular basis, so much the better. There has to be a REASON for people to get on the server, and even more than that: an OBLIGATION.
Hardly anyone is going to want to hop onto a multiplayer server if they think it's probably going to be empty, so they'll go play something else; so nobody ever goes on the server; it's a death spiral that happened on a private Minecraft server I am part of. For a long time, nearly a decade in fact, the host and the man who serves as the hub of our group of friends streamed random video game content every single Friday (mostly just for the dozen or so of us) for about six or so hours. When our server went online, he transitioned to almost always streaming THAT. So on any given Friday, everyone in our group knew we could hop onto the server during those hours, and at least he would be there. It created a common time when sometimes there were five or six of us all gathering together, where we could work on shit. We could also talk about maybe doing this again over the weekend. But after about a year and a half of this, he stopped streaming regularly (maybe it's once a month, now), cutting back on his video game uploads altogether. There are a lot of reasons for this, that I won't get into. But the point is, now nobody knew when to go onto the server. So traffic gradually declined, and maybe once a week I see someone on there for a few minutes; it's mostly just me because I started with singleplayer originally and actually autistically unironically enjoy the game as more than just a socializing method.
Perhaps what you need to do is make yourself that hub. Stream the game on a regular schedule (twitch or youtube or wherever), and make sure to tell people that's a good time to get on the server. Stick to it: don't miss even a single week, even if you sometimes end up by yourself for hours. You can't just hope for a gathering to happen by itself; you have to MAKE it happen.