Typical problems is to get as much throughput as possible through a fiber cable, at a distance as long as possible. That is the kind of projects that they typically do.
Another one: As you can send more data over fiber compared to copper and if you have planned to sell the line for comercial use, that means you can earn more money for a fiber line. However, the equipment is costly, so you must compare the overall cost and decide if you go for copper or fiber.