My best guess is what Freeman Dyson said in his autobiography Disturbing the Universe: he stopped working on nuclear reactors after TRIGA (01958) and Project Orion (01957 to 01961) because it stopped being fun. It became a question of bureaucracy and national security and empire-building and whatnot.

So people like Dyson, who learned calculus by spending his Christmas vacation working his way through a textbok for fun, were no longer available to design reactors. So progress on, for example, thorium reactors ended in the US in 01966 (thorium BWR is from 01960, thorium LWBR is from 01962, thorium MSR is from 01964, thorium HTGR is from 01966). In other countries it took a few more years. Teller, who worked on TRIGA with Dyson, seems to have stopped working on reactor design in 01960. The elves left Middle Earth. So hospitals still use TRIGA today for nuclear medicine.

I don't see the key issue here as being that Dyson and Teller were brilliant, though they were brilliant. Rather, it's that Dyson and Teller were curious and playful. I think curious and playful people exploring the possibilities of nuclear reactors will come up with many improvements, even if they are the ordinary kind of stupid people. It might take them five times as long as it would have taken people like Dyson, and there might be more accidental deaths along the way, but they will get there. But today the occasional curious and playful person who tries to investigate nuclear reactors is likely to get arrested, even if they pose less risk to their neighbors than Marie Curie: https://en.wikipedia.org/wiki/Richard_Handl (or assassinated by the Mossad: https://en.wikipedia.org/wiki/Assassination_of_Iranian_nucle...) and so the Navy is still using reactors very similar to the ones they used 60 years ago.

I see questions like "the technology available has improved, so why have costs risen?" as symptomatic of the worldview that anything can be bought. Teller, Dyson, and Handl were not optimizing their life decisions to maximize their earning power; they were curious about the world and wanted to preserve liberal democracy. You can't buy that. If you announce that you are going to spend a lot of money on nuclear reactor development, it will attract people who optimize their life decisions to maximize their earning power, not people who are curious about the world.

You maximize your earning power by owning things, not by learning things or figuring things out. People who optimize their life decisions to maximize their earning power will be no good at ferreting out possible improvements that can be made to nuclear reactors, because you don't get paid any more for making breakthroughs than you do for just plodding along.

Less, in fact, because nine tenths of the time when you're doing the kind of things that lead to breakthroughs, the things you try don't work, so you aren't delivering anything of value to anybody.

The other issue is that "Why can't we build nuclear power plants?" is a subset of "Why can't we build?" and I think the answer to that is basically that people aren't free to build. But nuclear power plants are probably the kind of building that people are least free to build.