How about because spent nuclear fuel will be hazardous to humans for the next ~20 thousand years? How do you amortize that cost? You can't just assume someone else will deal with it and call that cost savings. People talk about burying it but in reality it sits in containment vessels above ground and the more there is the higher the cost to deal with it so the less likely it ever will be dealt with.

Isn't that only applicable for Uranium 235 based reactors? Thorium is converted to Uranium 233 and when split the byproducts have an half life of 10s of years, meaning that the radioactivity drops to safe levels in "only" few hundred years.

This is much more manageable.

Anyway, that is to say that nuclear is a spectrum, and the current mainstream tech I believe it is the one that won because of the military applications (and therefore funding) back in the cold-war era.