That's because every change to a nuclear plant requires an act of congress. I'm serious.
In 2011-2012, ASME re-defined SA105 rolled steel. Arbitrarily, and the change was nonsensical. Millions of dollars worth of steel being used in in construction of Vogtle was suddenly no longer useable, and the design requirements for aspects of the plant required SA105 steel that was no longer possible to forge.
Vendors requested deviation to a superior steel to meet the delivery requirements, but WEC had to get congressional approval, since it deviated from the design plan.
Every ASME change, every NRC change, every shift in political tides that changes anything in the nuclear world in turn causes massive ripples, extraordinary costs, and delays. Nuclear doesn't work because the NRC, ASME, EPA, and D.C. make sure it's too expensive, over-regulated, and shift requirements throughout construction and operation.