In large part because of constantly shifting requirements, obstructionism, and various forms of lawfare. The US has become terribly inefficient at large infrastructural projects, see the CA HSR project which is massively delayed and over budget. China does not have this problem, and has been rapidly expanding their nuclear capacity.
A standardized reactor design and regulatory certainty would enable the economies of scale and accrual of institutional knowledge to efficiently build new power plants. In response to the 1973 oil crisis, France massively expanded their fleet of nuclear reactors which were producing more than 70% of their electricity in about 15 years. If the political will is there, it can be done.
There was no obstructionism or changing requirements to blame with Vogtle. It was all self-inflicted delays and costs by the designers and the builders.
France is experiencing exactly the same long delays and cost overruns today at Flamanville, despite having a welcoming regulatory environment, etc.
China also has lots of delays, and presumably cost overruns, but costs are a hard thing to pin down in China. China is barely expanding their nuclear program, only something like 50 new reactors are planned, a few orders of magnitude smaller than their plans for solar and wind.
CA HSR has experienced the obstructionism that Vogtle did not, but is still making good progress. The media narrative doesn't cover it well, but there are new sections completed all the time.
We have problems with big construction projects in the US, but nuclear takes those problems to the next level. And we have great non-construction alternatives for nuclear.
Gemini also qualifies as a large infrastructural project; it was delayed by concerns about the tortoises living in the area and challenges from local environmental groups.
China does similar projects in about 6 months. IOW, China does solar projects about 8X as fast as the US. OTOH it does nuclear projects in about 5 years, about 4X as fast as the US.
A standardized reactor design and regulatory certainty would enable the economies of scale and accrual of institutional knowledge to efficiently build new power plants. In response to the 1973 oil crisis, France massively expanded their fleet of nuclear reactors which were producing more than 70% of their electricity in about 15 years. If the political will is there, it can be done.