It's really not easy to uproot and move from an economically depressed area to an active metropolis. I've done it (Athens, GA to San Francisco in late 2012) and it was incredibly expensive and involved a large amount of personal credit use, cash assistance from family, and time. It was a huge stretch at the time for me to move across the country and break into a new industry, and I did it as a single man, willing and able to sleep on couches and occasionally a cheap hotel room (which left me going to the next job interview itching with flea bites). I spent a lot of days scrambling to figure out where I was sleeping that night while I navigated the terribly tight housing market in San Francisco. I would have never been able to do that with any sort of family.
If you are coming from a town where $28,000 is a really good salary for a job and your rent (as a single person with roommates) might be $250-$300 a month, how on earth do you save up enough to get two months rent in SF or even Oakland? Especially considering how little savings most families have in America, and that's when you haven't been laid off.
When you have certain highly fungible skills moving around is relatively easy. Now that I'm a programmer I can theoretically get a job in pretty much any American city I choose, and probably negotiate a starting/moving bonus so I'm not out-of-pocket for any costs associated with moving. That's true for all of my former coworkers and a lot of my friends out here in California, so I think it's easy to forget how abnormal that is.