A lot of jams are specific to communities and simply use Itchio as a submission platform. Like QaziJam9.5 happening right now is for QaziTV twitch viewers, not the general Itch community.
Having limited slots for jams would be pointless for jams like his. In fact they'd hurt jams like his, he'd probably just start using gamejolt instead, along with many other jam organizers that don't want to change their scheduling to match some randomly voting users who aren't even part of their jam community.