It's less that we've lost a technological capacity, and more than we are no longer leveraging economies of scale. France and the US built a lot of nuclear power

It's less that we've lost a technological capacity, and more than we are no longer leveraging economies of scale. France and the US built a lot of nuclear power plants in the 1960s, 1970s, and 1980s (well, mostly just France in the 1980s). Nuclear power plants often require specialized parts that aren't found in any other industry. That means when one-off plants are built an entire portion of the supply chain needs to be rebuilt, but when a serial run of plants are constructed (as was done in France) overhead costs of setting up the supply chain are spread out across all plants.