Is that because every country fails to adequately charge oil/coal/etc for its externalities? Are we essentially subsidizing oil, bringing the costs artificially low such that nuclear can't compete?
Yeah, let's not forget defence budgets that are spent to prop up oil availability too.
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.