I realize thats a common idea. Similar to the carburetor that gets a car 300 miles a gallon. "They" won't let it be sold.
There are so few ongoing nuke plant projects in this country I doubt there would need to be any conspiracy to stop thorium. But I can't see anyone trying to stop it. Why would GE or a company that makes nuclear technology want to stop thorium? Why would their potential customers, like Duke Power or Progress Energy, won't to stop it? Why would the companies who mine uranium not also want to mine thorium? Oil companies? I don't think oil companies think they are in competition with nuclear power. At least in this country, we don't have a lot of power plants that run off of oil. Maybe 17%? And I bet those are mostly backups.
My personal take, and I'm being completely cynical, is that the entities you mentioned above won't make any kind of move towards thorium UNTIL they can figure out a way to justify maintaining current profit levels...even though they'll be able to do it for cheaper.