“People are wanting to live closer to where they work,” White said. “To build street-level retail, you need residences. To attract residences, you need street-level retail.”
It's why it can be hard to jumpstart a neighborhood. People aren't going to live in an urban area which doesn't have the kind of retail you need to make an urban area attractive, but you can't support that kind of retail until you have the people.
With sufficient local population you have a viable local urban economy and a walkable neighborhood which then makes mass transit more viable as well.