The main reason is to allow people to submit their zines themselves if they want them included. Unless I've missed an option somewhere I believe Collections can only be added to by the creator. I'd rather people be able to choose whether they opt in to this as opposed to it being my own personally curated list.
Most of the zinequest releases are also new creations so I think a jam is appropriate in that sense, it's just that they're KS first with itch second.