Good question. I think making it available to all games is one way to make a challenge successful, though that's not the one and only key. Not having overly complicated rules are another thing, and having an attractive and easy to read format helps. Being an active OP is also a plus, you can ask NecrumWarrior about that. The monotype challenge has been doing much better since he took control of it, and it shows. Having a new and interesting concept is good too, but sometimes the concept might be a bit too restrictive or strange for some. For example, my Favorite Challenge is only for gen V, but it's still pretty popular since you get to use your, well, favorites.
Challenges that are too similar to something else don't always seem to go over well either, but I really think the main key is to not make a challenge too overly complicated. It needs to be explained well and easy to follow and appeal to people (which, if you write and explain things well and are persuasive, you can easily get people to want to participate I think). Also, challenges that rely on other challengers don't go well either, so trade challenges, tourney-style (though we don't have any, and it's clear why), rivaling other members, etc don't go over well. Same with challenges that only allow the OP to assign Pokemon. That's too restrictive, plus we don't know if they'll stay or not, cause then who else is gonna assign things?
That's just from my observations anyway. :) I think this post has gotten a bit too long. XD