I really can't offer much input as far as how to keep this challenge going- the main reason I like it so much is that it throws a lot of topics my way that I wouldn't pick on my own, focuses on full narrative illustration rather than production art, and with the deadline is forcing me to really churn this stuff out- fun aside, the only incentive I need is that I'm making things to put in my portfolio while in school.
So yeah... I can't really add to what's been said about how to keep interest in this going, since I feel like "excuse to make art" is enough :P. I don't agree that there needs to be a financial incentive- all those topics on CA gather tons of attention without any such thing. I will say that, for me, probably the most useful part of the storytellers challenge is the preliminary work stage- research, thumbnails, studies, etc. If we can't keep it up in its current form, I'd really at least like to see an ongoing discussion thread where we can take topics and post all those things for it, without necessarily taking it to finish or having a vote at the end.
Although really, who doesn't like a good challenge? :P