Nuclear has several major cost problems if it's own. Insuring against a major accident is prohibitively expensive, the kind of inspection+maintenance regime that's necessary for safety isn't amenable to cost reductions. The list goes on. Basically:

Nuclear isn't similar to the things that grow cheaper as the operators gain experience.

Much if fhe inspections and standards aren't "necessary for safety" but the direct product of paranoia and lobbying. Nuclear is brutally overregulated, far, far beyond what's necessary to ensure safety, and that's where most costs come from.