I'm sure with some extra R&D people could make those reactors reliable, but if you search the new generation reactors on Wikipedia, you'll see every one of them ended in some kind of failure.
I'll wager this was mostly because of the lack of funding. Those reactors were not as thoroughly researched as the light water ones. In a sense, they're still stuck in the 60's. (Really, putting molten salt reactors in the "new generation" bucket is a bit of a joke. It's old stuff that was shut down mostly for military and economic reasons, and only recently got more traction.)
Another possible cause is path dependence. We have a whole industry around solid fuel, light water reactors. Switching to molten salt throws much of that away.
Much research still need to be done. We may want cleaner salts, we want even less radioactive waste, and we definitely want to accurately assess the safety of this stuff ("passive security" is not enough by itself). This means more prototypes reactors, of various sizes, each more expensive than before. We want a few million dollars for the small ones, then a few billions for the big ones.
I was serious when I said I was sure some extra R&D would solve those problems.
It may not be just lack of founding. Materials science has advanced a lot since then, and those problems look solvable now, but I'm not sure they could be solved by the 60's.
I'm sure with some extra R&D people could make those reactors reliable, but if you search the new generation reactors on Wikipedia, you'll see every one of them ended in some kind of failure.