> proven extremely difficult to build and operate and tend to leak molten salt

The only are 2 molten salt reactor ever built and neither one had a leaking problem. In fact, they would even be self sealing as the salt will cool down rapidly and seal the hole. If you have a rupture in the whole core, the spilled out salt would be captured in a pool and harden, it would be contained in the nuclear site boundary. This is the advantage of high temperature and salt. The temperature differential to the environment that the cooldown is very rapid.

Compared to PWR, the danger of a a molten salt reactor is incredibly small. There is no high pressure and steam, thus no explosion that can transport stuff like zirconium outside of the nuclear boundary.

Its also false that they have been extremely difficult to build. The Molten Salt Reactor experiment was operated and built by small time at a tiny fraction of the budget that were spent on many of the other nuclear reactors built at that time. They are far easier to build then a PWR in terms of complexity of and scale involved. The Molten Salt reactor experiment (the first ever molten salt reactor) was one of the fastest nuclear reactor was ever built.

The reason we don't have them has more to do with the history of nuclear technology. The Navy focused on PWR for ships (as makes sense=, those were commercialized with big investment of government and industry and were the most mature by far. Molten Salt reactors were in development by the Navy and were discontinued when rocket technology made nuclear bombers unneeded. After that the inventor of molten salt reactors only got a tiny amount of money to continue his research for a civilian reactor.

The big nuclear players didn't really care about building new reactor types as they had lots and lots of orders of the production designs they already had. There were only a very small number of people who were educated about what a molten salt reactor or how it worked. It required a somewhat different skill set (the inventor was a chemist) and neither government nor industry had a taste for commercialization at that moment. It didn't help that the inventor basically made himself unpopular with the regulatory agency when he claimed PWR were unsuited for civilian use.

When regulation changed because of TMI and Chernoyl the nuclear companies had their hands full with rebuilding all the plants according to new regulation to keeping the existing fleet ruining (and finishing project that were being built) and by that time the regulation had changed so that it was basically impossible for any new type of reactor to be licensed. In the US the technology was hard coded in to the regulation and made it impossible to deploy a new type of reactor in a commercial way. Since then hardly any new reactor type has been built or tested as you can not get fuel for anything beyond a tiny research scale reactor without passing full commercial regulation.

Development of pretty much all serious molten salt fuel has now moved to Canada as the regulators there have not hard coded specific technologies and the regulator investing its own resources to get threw regulation of such a reactor. In the US, you would have to pay the regulator to evaluate your design with no timeline or guarantee that would even allow it at all. Meaning you would have to design a full commercial reactor before the regulators even tell what you would need to do to convince them about the reactor. Meaning you potentially spend 10s of millions on paying the regulator before the actual regulatory process could even start, as you would have to finance the development of a regulatory framework for molten salt reactors.

Thankfully the DoE has realized this now and they are working hard at chaining their approach and congress is for once actually engaging in a bipartisan effort to improve the situation. See this talk for more information about government efforts: https://www.youtube.com/watch?v=p1lkDRX2huM