For me it’s the social aspect, I could set myself a two week challenge at any time but what makes it a jam is doing it with other people. My fave jam (sadly discontinued) was small and only ran once a year, but the hosts were chatty and supportive and we saw the same faces year after year so it was lovely. Still hunting for something to replace it.
Meanwhile I joined two new jams recently. They were both hosted through Discord, which is fine. But the first one forgot to make a channel for the jam for the first few days so we were adrift in this massive spammy server, and even once the channel was set up the participants were basically left on their own. The second one the hosts were just completely silent the entire time, even when directly pinged, until the deadline when they said a quick congrats and that was that.
You don’t have to be the world’s biggest extrovert to host a jam, but give people something. Cheer on the participants, leave a heart emoji on someone’s progress screenshot, in one recent jam I encouraged the first-timers to write a daily devlog with me on the Itch forums and that was really fun. Just take an interest, basically is what I’m saying here.