(Scott, did you intend to make it a "registrants only" discussion? If so, I'm sorry.)
I think there are some unpleasant things going on in our neighborhood groups, and that's definitely part of the problem.
It can be hard for people who are deeply involved in something to make it feel inviting for new folks, or even to see that it's not inviting for new folks. These are things we could all work on.
I think another factor is that lots of people get excited about working on a particular project or issue, but they're not interested in the work of maintaining a standing organization. Life is short, we've all got busy lives, decisions about bylaws feel pretty distant from the playground project you were so passionate about… So once the playground is built, the trees are planted, the political campaign is over, or whatever — people drift off. (This doesn't mean you lose the connections forged during the project, though… I think the Hillman City group actually leaves behind a pretty good network of relationships and an e-mail list that will continue to serve the neighborhood, even without an ongoing organization. And if the need should arise for an official group, I have no doubt it will rise again, partly because people know each other now, and how to work together.)
They say that the era of civic-groups-as-social-glue is waning (see Robert Putnam, Bowling Alone). People are getting their sense of identity, belonging, connection, etc. elsewhere (church groups, activity clubs, school communities, even at work). Many of us are feeling these pulls from too many directions at once, even (who, me?). And so maybe when the neighborhood group meetings start to feel less than fun, it makes sense to bail & put your energy somewhere else.
(Which wouldn't necessarily be a problem, except that these neighborhood groups often represent the "voice" of the neighborhood, the official channel of communication with the City… so if people drift off or otherwise bail, they're no longer part of that communication…)
I'm definitely *not* a major participant in any of the groups we're talking about, so I'm not presuming to have any comprehensive answers. But these are a few of the factors that affect me, and possibly others.
Thanks for bringing this up, Scott.