My current hypothesis is that nuclear became politically toxic in the 80s, and public will to support nuclear research evaporated, leading to everyone thinking "nuclear == light water reactor", and the entire field of research on fission-based nuclear stagnated as a result.

I'm not an expert though, just interested in the topic. I read up quite a bit on MSR and LFTR a decade ago and concluded that it was really interesting tech that can absolutely work but just never got enough interest for someone to "go big" with it. The wikipedia article[0] has a recent developments section that suggests the tech is quite viable.

[0]: https://en.wikipedia.org/wiki/Liquid_fluoride_thorium_reacto...