Angie Schmitt at Streetsblog thinks that the reason is that much of the decision making ultimately remains with the previous generations who aren't as keen as their younger counterparts on city living.
The Streetsblog piece links to a longer form reflection by The Urbanophile. Aaron Renn writes:
"I don't see any signs of the older generations getting through the grieving process and moving on. This makes me think that for us to fully embrace a true urban policy, even in city government itself, it is going to take generational turnover."
Thanks to Jason Van Patten