The generic problem is that nuclear plants have to be big to amortize fixed costs (like personnel to operate and guard them), but if they're big they have lots of parts and more connections between parts, so to make them sufficiently reliable they need expensive careful construction. The more welds a NPP has, the more reliable each individual weld must be.

In contrast, something like a solar field, while it has lots of parts, doesn't depend on all those parts working for the field to work. It's by its nature highly redundant and fault tolerant. It also doesn't require supervision anywhere near the same extent as a nuclear plant.