One part this, two parts the economics of a novel technology platform being deployed in a large size, three parts American labor costs and inexperience with megaprojects.
Similar to why we can't build ships [1]: high input costs, notably materials and labour, and a coddled industry that is internationally uncompetitive. With ships, it's the Jones Act and shipyard protectionism; with civilian nukes, it's misguided greenies. (Would note that we're perfectly capable of nuclear production if it happens under the military.)
[1] https://open.substack.com/pub/constructionphysics/p/why-cant...