It's really simple, summed up by the slogan: "Better houses, Easier on the Planet". Our knowledge of what makes a good building, and what it's impacts on the environment are, has grown tremendously. While the initial impetus was driven by environmentally minded people, the "Green building" movement has now reached the mainstream, because its ideas make sense to many people.
Homeowners are finding the homes to be of higher quality and having lower operation and maintenance costs.
This is why we say "Building Homes that cost 50% less to run and maintain"