Short answer–because all of the political, technological, and financial focus was on the liquid-metal fast breeder reactor. Later on, due to fears about non-proliferation, the US cancelled plans to commercially reprocess spent nuclear fuel to extract plutonium, and the case for the fast breeder reactor was toast. Because there were no fast breeder reactors to take all the plutonium that had been generated from light-water reactors, in 1982 the US government passed the Nuclear Waste Policy Act and started collecting a tax that would be intended to pay for what would eventually become Yucca Mountain.
Read and comment at: http://energyfromthorium.com/2011/12/23/techtalk-why-tmsr/
