Hard to say, I can only make smart guesses for the reason of lacking participants. I have thoughts that holidays are best time for game jams cause people have more time to make something for game jams.
However, my best guess would be youtube followers or viewers. GMTK Jam had 9000+ entries while Juniper Game Dev Jam had 3000+ entries. That's 9x and 3x more (respectively) than Ludum Dare which is held regularly. So yeah, probably reputation does have impact on participants number.