There are a few reasons why, but most of it likely stems from a lack of knowledge about solar energy and passive efficiency. Nonrenewable energy is also still relatively cheap, and installing passive systems may initially be more expensive or cumbersome, so many builders and homeowners skip over this part of the design process in favor of a quick, affordable build.
Karyna McLaren, who works for a company that designs, produces and installs solar energy systems, explained it this way: “The upfront costs can sometimes be prohibitive and often discourage homeowners to consider implementing a system. This, paired with the 'cookie cutter' approach to home building that has become wildly popular, means customizations are more difficult to implement.”
The more homeowners continue to learn about passive solar energy and the more they demand it from the market, the more we should see it adopted as a standard in architecture.