I believe that the return to the big city has at least 3 explanations:
1. Job markets are tougher these days than they have been in previous decades. As I recently wrote, big cities like New York City offer more job opportunities than the country side and the suburbs (drivebycuriosity). Big cities also provide more and better possibilities for education (universities et.al.) which might be helpful for finding jobs.
2. Gasoline is much more expensive than in the last century. Before the year 2000 a gallon of gas cost below $2 at the pump, now drivers have to pay close to $4 (gasprice). People, who have to drive large distances for commuting to their workplaces, now have to pay a much larger part of their income for gas. Hence it makes sense to move to a dense city with short commuting distances and an affordable public transportion system such as in New York City.
3. It seems that big cities also benefit from another trend. Today people spend smaller parts of their income for cars, furniture and other expensive things and more for live events because they want to experience something. Big cities supply much more leisure facilities than the countryside like rock concerts, clubs, operas, theaters, museums. Big cities also offer more chances to meet other people and to acquire new friends - with much lower travel costs.
Welcome back to the big city!