Building new reactors is costly and you need public opinion to agree to that. Unfortunately when someone proposes building new kinds of reactors some asshole backed by "Oil Corp Co." runs ahead screaming "Chernobyl Fukushima!!!" and public opinion is instantly biased negatively to that idea.

Its completely illogical to block Nuclear Power advancement, but goverments are either lobbed or public opinion disagrees after being fed complete horseshit disinformation.