At the time the USA and USSR were locked in a nuclear cold war and mass producing nuclear weapons was a priority.
Even though the Thorium fuelled MSR was clearly superior in every aspect, efficiency, safety, low waste production, size and cost of operation. It was abandoned in favor of the Uranium fuelled LMFBR (Liquid-Metal Fast Breeder Reactor) because the latter could produce lots of weapons grade nuclear material, while the MSR could not.
There was talk of internal strife as well, the head of the AEC (Atomic Energy Commission) at the time viewed the LMFBR as his pet project.
In short order, the MSR, and thorium were given highly unfair reviews that painted the MSR in very poor light compared to the LMFBR. Which we know today as totally untrue.
As a result the MSR and it’s wondrous fuel were buried. Never to be spoken of again except by the very few who were involved in it’s development…. until now.