No country has seriously invested in the thorium fuel cycle because it cannot be used to create weapons. Unfortunately, the technology also began to look most promising as an energy source around the same time the Three Mile Island nuclear accident effectively ended all interest in nuclear energy in the United States.

India has shown some of the most interest to date, due to their lack of domestic uranium reserves. But it's been slow going their fast breeder reactor plans were delayed by like two decades. But it is built and it was loaded with fuel last month [0]

The French interest in breeder reactors and nuclear reprocessing also originates from a similar concern about lack of domestic access to raw uranium. Though Super-phoenix [0] was a more traditional uranium -> plutonium approach and not thorium. They gave up because just using uranium is way, way cheaper than synthesizing your own fissile materials.

[0] https://www.world-nuclear-news.org/articles/indias-prototype...

[1] https://en.wikipedia.org/wiki/Superph%C3%A9nix

Thorium can be used to make weapons via the breeding cycle. It's much less convenient and straightforward than uranium/plutonium, but it is possible.

Also, it’s only energy positive under some specific carefully managed conditions, and is a real pain to make work.

If you have easy access to uranium, you just use it directly instead.