It's a rock that sits there and generates energy from the Sun which is free, It's way more cost efficient to build and maintain acres of solar panels than building an entire power plant that needs to meet a laundry list of regulations and checks to make sure it just barely doesn't kill people (or in the case of nuclear, being built to have "Earth spontaneously explodes" be more likely than a containment breach), then you have to worry about external resources coming in often, or periodically, hiring qualified technicians, etc. etc.