There's the "we forgot" hypothesis, but I think a more realistic hypothesis is the "we got too rich" hypothesis.

Construction productivity has stayed stagnant for more than half a century, while manufacturing productivity has sky rocketed and made us all fabulously wealthy compared to when the first nuclear reactors were built half a century ago.

I don't trust China's public cost numbers as much as I trust their actual capital allocation on the grid. And I will trust GE's numbers once they have actually produced something at those numbers, as pre-build cost estimates for nuclear are not believable due to their extensive track record.