See this account of a developer's struggle to gain approval for I'On, a New Urbanist community in Mount Pleasant, South Carolina, just outside Charleston:
[T]he neighborhood is located in close proximity to two historic districts that are, if price is any measure, the most sought after places to live in the area; through its Master Plan and Strategic Plan the Town had adopted a clear vision for the kind of development they wanted; we had two of the best, if not the best, planning teams in the country creating the initial plan [Dover Kohl and Andres Duany's DPZ - AC]; no less than four environmentally oriented groups endorsed the plan along with a substantial number of community leaders; and the developer had a track record of successful TND development within a 90-minute drive of the subject property.
But to gain approval, the developer had to cut the project from 800 single-family homes, 440 multi-family units, and 90,000 sf of commercial space to 749 single-family units and 30,000 sf of commercial space and eliminate the multi-family units, a gross density of only 3.1 units/acre. It took two city council votes spaced over two years and a lawsuit appealed all the way to the South Carolina Supreme Court; the council members who voted for the project were turned out of office after bitter elections. Attempts to connect the development to surrounding neighborhoods were shot down. The developer won the right to locate a Montessori school in the village center only after obtaining another judgment affirmed by the court of appeals.
Although one can speculate why there are not more New Urbanist greenfield developments, this kind of story, I think, explains why we don't see more NU infill developments.