If modding involves sending assignments to an individual or tracking them in some way, then every additional participant represents additional work on the part of the mod. Personally, I don't want to expend the energy (and my time/energy participation in fandom is limited, even as a normal person) on people who aren't going to turn up with a story (or whatever the exchange is intended to produce.) This is especially true for challenges where the mod has to generate their own object for participants to work off of (e.g. for picfor1000 or any of the bingo card challenges), or for challenges where there are assigned posting dates so that people watching the challenge are getting a story/icons/meta, etc. every day, because that sort of posting still requires pinch hitters.

I mean, I won't say this is universally true, but the reason for a lot of challenges is so that the mod can have more X exist in the world, and I can do about this much to get that much X, and people who don't turn X in mess with my X production formula, you know? And I, personally, am not running 3_ships because administrating challenges is something I enjoy doing.