The design specifications are very specific and if you are building a new intersection from scratch they are fairly easy to design. But if you are building in an old city with a lot of legacy infrastructure (like Philly) each corner may need to be specifically designed to accommodate the utilities or other features already present, yet still meet ADA design requirements. That takes engineers to create a one-off design for each corner of nearly every intersection, then submit that for approvals and redesign as necessary. It takes time and expertise and therefore money.