I'm nit sure but my guess is that if it's a big name jam (hosted by Github, GMTK jam where the servers crash of popularity, another edition of a semi-popular jam, etc.), it will be more likely to be featured.
Yeah, I understand that big-name jams like GMTK or Game Off (even GameOff had lesser numbers than my jam before it was featured and now they have x4 more) are often featured — that totally makes sense.
But I’ve also noticed that sometimes much smaller jams (with 10–50 participants and starting in just a few days) get featured too.
So it looks like there might be some additional criteria besides popularity — maybe activity, theme relevance, or how the jam is presented on the page?