I personally think the reason that solar and wind have gotten so cheap so fast is that they've found a way to manufacture small power plants than can be easily deployed. The more traditional forms of power generation such as nuclear, coal, gas, and hydro-electric seem to always focus on building huge generating stations, and building everything from the ground each time. With solar and wind power, the design problem is already solved, and you can (relatively) easily deploy a small power plant and build on as demand grows.
Nuclear power plants cost in the billions and take many years to get up and running. If they found a smaller, more standardized reactor that could be more easily deployed, it would be a lot cheaper to do, and they could build out capacity as they need it instead of having to plan 20 years in the future.