Featured cities are selected by our Marketing department based on several different factors. Foremost is the average number of listings per month (Atlanta has 30-40, you're probably not seeing them if your outside the US) and how many company pages are in an area.
Then there's secondary considerations like having a popular tech market, how much business a city generates, traffic on SO from a region, etcetera.
Third, I think we're on hold with adding more cities. If we added more, the UI would have to be re-worked because then the list of cities would become a little unmanageable if it grew from 20 to say 50 cities.