I've made games for some jams already and my guess about what makes a jam popular would be: community. Look at jams that take place once a year (especially featured ones), and how many people join each year. It would sure be interesting to also have insight on itch.io's traffic to make better sense of the numbers, but nevertheless, you'll notice jams that get increasingly popular year after year, or month after month (like indipocalypse). You might want to contact their organizers to ask them about their experience.
Although I don't know if that's the right thing to do, if I started a gamejam, i'd do everything I can to bring its participants together, make the jam a regular and consistent event and celebrate the creations that made it to the top (congratulations, youtube video, discord roles, help with whatever you do best, etc).
I hope that helps.
Cheers!