More importantly, poor people in New Orleans had virtually no financial capital; instead, they had very rich social capital. Networks of friends, family, and neighbors substituted for things like credit and savings when they had personal or financial problems. In a strange city, with no network and no financial resources, they feel incredibly helpless and exposed--with good reason.
Those are the same reasons that many people in declining rural and industrial cities stay where they are. When I was doing an article on the economy of upstate New York, one of the most striking things I noticed was that people who talked about moving in search of better jobs almost all had at least one close relative who had already left. The people who were afraid to go were the ones who didn't know anyone outside of Buffalo, or Rochester, or Syracuse, or Wayne County. That's a problem it would be pretty hard for the government to fix.
Network migration, what a novel concept! I'm jealous that Ms. McArdle makes crystal clear in one post what I've struggled to articulate since I started this blog. The world is indeed flat for those lucky few who benefit from a diaspora of tacit knowledge.