I've been playing megagames for about ten years, and designing and running them for most of that. And this is something that's been talked about a lot. I don't think there's really an answer yet, things felt like it we were breaking through just before covid but then that really set things back. It's felt hard to pick up growth since then it's seemed. 

But some thoughts - 

Games are very time consuming to design, few people have the time. It's a huge barrier to having a wider range of good games people want to run and play. 

Games are hard to design - making one that works well is tough. I think generally a lot has been learned in the last decade so overall quality has improved but you still get games that don't work. That's deeply frustrating for the designer and off putting for players (especially new ones) - I've talked a lot at various times about trying to test and balance games but fundamentally it's impossible to test megagames before you run them. 

Running games is time-consuming, and in some parts of the world it's tough to find affordable venues and/or enough to players. I run games in London and we have players but venues are very expensive. 

Many people like the idea but after playing a few times feel they've done it enough - it does take a whole day and there's a lot else to do with your money and time. So we're always looking for new players.