Because 'Big-Uranium' bought up all the patents and made them secret.
You can't make a patent secret. That's the point of patents, we're not a system of trade secrets. Knowledge gets shared when the patent is approved even though exclusive rights are retained by the inventor for 20 years. I'm sure the majority of thorium reactor technology patents are expired since research ended so long ago. Now if the researchers never patented their research and kept it secret that's another matter.