You and I have had this discussion before. The vaunted "market forces" argument is full of holes. Developers, architects and engineers have a hard time forcing themselves out of  group think even for really good ideas. I often bring up the reluctance to embrace decisively superior, more efficient technology in favor of an old clunky paradigm. The example I often use is wasteful, inefficient Edison bulbs being swept off shelves for years while CFLs languished for nearly 20 years. Even super efficient, elegant LEDs had a hard time competing for a number of years in multiple applications.Green rooftops are just one more example of this approach avoidance to new ideas.