One of my favorite game designers in the video game realm is Spiderweb Software, who have done a great job in their dozen or so indie RPGs of giving players a good compelling reason to follow the plot, usually through violent coercion. It turns out that being oppressed by an evil empire is a good way to convince people to do things.

Interestingly, the group that I've had the best experiences with is a Discord server (this is where the Only War games happened). While I've generally played with the same few core players, the fact that there's this ongoing narrative has really helped to keep people pushing forward, because you can't just go off on your own to do what your group wants to do. Someone else will have something going on that is important. I think this is more or less analogous to what you describe.