Tl;dr: Because we haven't created a safety standard for it.
The lack of an Underwriters Laboratories (UL) standard is perhaps the biggest obstacle to the adoption of balcony solar. The company certifies the safety of thousands of household electrical products; according to Iowa State University, “every light bulb, lamp, or outlet purchased in the US usually has a UL symbol and says UL Listed.” This assures customers that the product follows nationally recognized guidelines and can be used without the risk of a fire or shock.