It takes a long time to build them. By the time they are built, the economics for them will be much worse than they already are relative to solar, wind, and hydro.
Nobody wants a plant in their backyard, nobody wants to store the waste in their state, the investment takes the length of career to pay off.
Too expensive
Because it is outrageously expensive compared to renewables, you can't turn it off at night, it takes literally decades to build, and nobody wants it near where they live.
And even once you have it, you still need storage and/or overbuilding, because demand itself is idiosyncratic.