As for my own stories, I've found that the number of people interested in contributing are affected by two things:
1. How large the story is. The larger the story has become, the more people seem interested in contributing.
2. How active I am. The more pages that are added to the story, the more it seems people want to contribute their own story segments.
For Example, "Glory Hole" hasnt seen a submission in a long time, but at the same time, I havent contributed anything to that in a long time, and and there really isnt a lot there.
On the flip side, "Futa Apocalypse" gets probably two PMs from people interested in contributing, and one or two actual submissions each week. Early on, it was just me writing for the most part, but over time, its drawn more contributions At the same time, I try to add a little something to it as I'm able, and there's a fair amount of material there for people to read as well.
I expect it all boils down to having enough material for people to get swept up in the story, and to update it often enough to keep people thinking about it.
Thats my two cents on the subject, and how I interpret what I've seen anyway.