Short answer: the military already knew uranium and how to work with it, and thorium was pretty much unknown.
Long answer: Even though Weinberg invented the Light Water Reactor (one of the most common reactors today), he was eventually removed from the nuclear industry by a cranky senator that got sick of Weinberg's insistence that there was a far safer way to go. Also, Nixon chipped in and wanted jobs moved to California, and so selected a Fast Breeder program for political gain. He admits publicly in the next video that he didn't do that well in science, and did not really understand what a breeder reactor was, let alone the differences in them!
This Google Tech talk features nuclear engineer Kirk Sorenson explaining what happened to Molten Salt Thorium Reactor funding, and includes video of Nixon deciding to fund the IFR over the MSR.
MORE HISTORY:-
Nixon instead funded the Integral Fast Breeder reactor, which resulted in a very good reactor, the EBR2. It eats nuclear waste and survived a number of tests similar in condition to Fukushima's total power failure!
The EBR2 total power failure tests: from "Pandora's Promise".
The EBR2 burned nuclear waste and worked well for decades.
https://en.wikipedia.org/wiki/Experimental_Breeder_Reactor_II
INTEGRAL FAST REACTORS (IFR)
The EBR2 was the world's first IFR, and it ran at normal atmospheric pressures. Chernobyl and Fukushima were as bad as they were because of high pressure water. Water reactors run at 150 to 246 atmospheres!
http://www.world-nuclear.org/info/Nuclear-Fuel-Cycle/Power-Reactors/Nuclear-Power-Reactors/
High pressure water in a nuclear reactor is not as safe as it could be, and caused Chernobyl and Fukushima. (But even these older technologies have served us far better than coal, saving many millions of lives over the decades!)
http://blogs.scientificamerican.com...o-fossil-fuels-may-save-up-to-7-million-more/
High pressure water = bad. Nixon funded the fast reactor program that used room pressure coolants, making reactors much safer. As the EBR2 video above shows, they have designed passive safety features that allow the reactor to cool itself. Older reactors like Chernobyl or Fukushima REQUIRE power to cool, newer reactors don't. In fact, they require power to WORK, and will automatically shut themselves down in a power failure, without Homer Simpson having to push a button or anything!
IFR's also have their own tragic story of underfunding. IFR's would have turned America's nuclear waste into about 1,500 years of fuel a and the UK's waste into 500 years of clean energy, but Clinton shut it down in 1994.
https://en.wikipedia.org/wiki/Integral_fast_reactor
Nevertheless, GE have based their PRISM reactor on the EBR2, and they are ready to build a commercial prototype in the first nation that lets them!
https://en.wikipedia.org/wiki/PRISM_(reactor)
THE MOLTEN SALT REACTOR (MSR)
The IFR's normal room pressure safety advantage comes with one (slight) disadvantage: it uses liquid sodium as a coolant. It's not under pressure, but it's also not chemically stable. It explodes if it hits water. So while the IFR is vastly better than a Light Water Reactor, and a Light Water reactor is
thousands of times safer than coal, the IFR is still not the best we can do. The Molten Salt Reactor is, which uses hot liquid salt and is a liquid fuelled reactor that *cannot* melt down, it is already a liquid, and in any power failure the liquid fuel just drains away to a safe drain tank where it automatically cools. In any power failure, gravity takes over. When was the last time gravity failed? ;-)
We could have gone straight to the MSR and avoided Chernobyl, Three Mile Island and Fukushima. Decades ago, JFK toured the MSR (there's Weinberg on his right.) But sadly, fate had other ideas, and we end up in a world dominated by climate change and coal's mercury poisoning and coal's radioactive fly ash waste in our lungs! Kirk Sorenson unpacks the full story of why Weinberg was side-lined in the very first video above.