Money. The average family can't afford to live a "good life" in the city. If you can afford a 2200 sq.ft. flat in the city, you can afford a 4000 sq. ft. house on a few acres in the suburbs. And you usually know who is walking by your house, where in the city you really don't. And, you usually want other kids around, and suburbs are full of kids. The schools are, on average, better. You can have a basement and a garage. If you live in the city without much money, you live next to a crack house, not near the park. You worry your kid will be abducted, or hit by a car. The list is very long. It's why people live in the city until they have kids, then move out. And when the kids are gone, they move back, unless they have grandchildren on the way.
Believe me, if I could consider living in the city, I would - parents don't live in the burbs because they're so darn fun. The trade-offs parents make are endless, and usually with the best interests of the kids in mind.