Most of the reactors we have in the US today are the result of building many of a few models per year during the late 1960's - early 1970's.

In 1974, 12 reactors entered commercial operation and 13 more were begun.

https://en.wikipedia.org/wiki/List_of_commercial_nuclear_rea...

The technology was given almost a century to prove itself cost-effective. It did not succeed in the market and only ever worked with truly massive amounts of government subsidies.

High fixed costs + more reactor sites -> more expensive, more nuclear waste sites for taxpayers to clean up