There's some remarkably rich history behind the reason why. Eugene Wigner, one of the early players in nuclear energy development got one of his studens, Alvin Weinberg interested in using Thorium as a nuclear fuel (while Enrico Fermi and the other lot went off for Plutonium)

In the late 50s Weinberg and co. found there was a funding opportunity for experimental reactor research from the US Air Force for a flying bomber. They thought, "nuclear powered bomber... that's really fucking dumb" but hey free money. In the process of that they had to make something that was so simple, so stable, so light that it could fit into a plane that could fly. So that meant atmospheric pressure operation with a coolant that doesn't wanna bounce on you. This was called the Aircraft Reactor Experiment and was the first demo of a type of molten salt reactor, ran at like almost 900 degrees Celsius and all that stuff.

Well, with the success of the ARE, Weinberg badgered the brass to get standalone funding for a design they slapped together and ended up working great. They got it, and the Molten Salt Reactor Experiment was born. They built it with fuck all funding and despite that, it worked exactly as they calculated as it ought to do, and ran for thousands of hours continuously, stable at some of the highest temperatures ever obtained by a nuclear fission reactor. There's even a part of the MSRE history where one of their probes dropped into the active molten fuel salt and they had to go fishing with a long pole and some fibre optics and nothing bad happened lol. Try losing a foreign metallic object in a pressurized solid fuel reactor lol.

Aaaanyway. At the end of the research part of the MSRE in the early 70s the people there were pretty chuffed about what they had accomplished, so Alvin Weinberg went to the Atomic Energy Commission to request more funding to make a full-scale commercial model. However a number of conspiring things occured so this did not happen. The golden child worshipped by the AEC was the Plutonium-fueled Liquid Metal Fast Breeder Reactor which had been favored from the outset by the US Government simply because there was more pre-existing experience with Plutonium at the time, and it didn't help that Weinberg was almost indirectly insulting the intelligence of much of the AEC, including Admiral Rickover who headed the Nautilus nuclear submarine project, by suggesting that light water reactors were not suited to civilian power generation, citing loss of coolant, containment and spreading of radionuclides into the environment. Weinberg happened to be one of the inventors of the light water reactor. Ouch. And to add on top of all this, the MSRE was in Tennessee, the LMFBR was in California, and Nixon wanted a jobs project to show to his home state. And so, the MSRE not only didn't get it's expansion funding, but was phased out entirely. And the only people who knew about it were maybe a few hundred in the area around Oak Ridge, and the idea faded for decades.

"Well, why didn't private industry take up the torch?" -- Westinghouse, GE and the like operate on a razor blades business model. They'll sell you the reactor pretty much at cost, but make their money back by locking you into a multi-decade Uranium pellet fuel contract. A reactor that costs little with the fuel being practically free doesn't fit in well to their business model. Not to mention it takes an inordinate amount of time and money to get a new design licensed commercially with the US NRC and no private company is going to willingly sit blue-balled while that happens and they have stockholders tapping their foot impatiently.

Thankfully there are many start-ups that are going the MSR route these days, not to mention the Chinese TMSR project. I'm mostly hyped about ThorCon which plan to make Thorium-fueled MSRs in Indonesian shipyards and export them globally by the early 2020s, led by one of the dudes in charge of building those huge Hellespont supertankers and shipping barges, and then there's Terrestrial Energy which is in the later stages of applying for a billion dollar grant from the US DoE for a commercialized version of a (Uranium fueled) MSR which they will then certify in Canada where the regulatory structure is a lot saner.