The reason is pretty obvious and doesn't require any kind of conspiracy.
Nuclear can be done safely, we know this, because every single time there has been an accident it's because an operator did something wrong.
The problem is that nobody has yet designed a reactor that a sufficiently amoral operator could not make unsafe. Even if you have completely automatic and passive safety features, a bad operator could disable them if a false positive happens even once and costs them money.
For this reason nuclear has a LOT of regulation and red tape. It has far more than any other kind of energy, because even though the risk of accident is low the outcome of an accident is worse than any other kind of energy except hydro. Hydro has fewer things that can go wrong that are cheaper to check however, so regulations there tend to not be as expensive.
Molten salt reactors manage to be expensive for perfectly normal engineering reasons though. The salt is highly corrosive, meaning you need tons and tons of extremely expensive piping that can withstand extremely high temperatures and extremely corrosive environments.
There's a reason none of _those_ have been built either. No amount of red tape would make them unviably expensive if the end product was cheap enough to run, but it isn't.
Also I should mention, part of what makes them passively safe is there is a plug at the bottom that melts if the reactor overheats. This is easy to bypass, put something over the plug that won't melt.
> There's a reason none of _those_ have been built either.
Let me introduce you to the Molten-Salt Reactor Experiment[1].
What you probably meant is that none have been built commercially. That is true, but again as I mentioned, not because of their technical drawbacks but because of politics. In fact, the inventor of the light water reactor, Alvin Weinberg[2], was a strong proponent of the molten salt reactor over his own invention. So strong that he fired was from ORNL because he was claiming that light water reactors are inherently unsafe and that MSR is a better design.
Nixon ultimately sacked him because he (Nixon) chose to support LMFBR (Liquid Metal Fast Breeder Reactor) because it was being built in California, and in return he got political support that he needed. MSR ultimately lost due to pork-barrelling.
> This is easy to bypass, put something over the plug that won't melt.
I mean you're shifting goalposts here. The "operator" has a specific meaning - someone controlling the reactor from the control room. They don't have access to the freeze plug during normal reactor operation.
But even if they did do what you're suggesting, the pressures inside the MSR are so low (on the order of couple of bars) that the damage would be quite limited.
We had a working reactor in the 1960s, but we chose not to develop it commercially due to our inability to choose rationally. That's why we are where we are now.
It's part of a larger trend of eroding competence at civic infrastructure scale construction, but also specific to nuclear we've found repeatedly that construction and decommissioning costs and schedules were wildly optimistic.
Plutonium-239 (just an example) has a halflife of 24,000 years, ground water is a thing and it moves a lot. Modern concrete is both water permeable and has a tendency to degrade aggressively in wet environments over alarmingly short periods of time. Try again?
That's not a technical issue though.
Cost efficiency of nuclear energy has actually declined over time...which, unless science is devolving, should tell you something hinky is happening.