People have been gravitating to bigger cities, and one factor that's driving it is two earner households. It used to be that if the husband had to move for a new job, he'd pick up the family and go. This is much less possible than before, since the wife needs to find a job too. If both of them work in specialized occupations, only big cities are likely to have appropriate jobs for both. This makes land in larger cities at an increasingly large premium.