Increased nuclear safety. Proponents often tout nuclear's relative safety. Good, but that safety is hard won through regulation. Compliance is costly, and improved safety has been subject to diminishing returns.

Economies of scale. Building a nuclear power plant is a massive undertaking, so the world simply hasn't been building a lot of nuclear power plants. Supply chains (parts, know how) are getting smaller and more uncompetitive with every passing year as other forms of energy production have become more attractive.