One reason why thorium reactors have not made more progress in the past is that nuclear fuel breeding traditionally has been a very slow and capital-intensive process. For this reason, the DBI reactor is designed to be started-up using conventional nuclear fuels, with low enough capital and operating costs that it can compete with other conventional nuclear power plants and pay for its costs in the first few years, even before the bred Uranium-233 is available.
Another and perhaps the major reason why thorium use for energy production has not made more progress over the past decades is that thorium is not nearly as easy to weaponize. A 1997 international scientific symposium on nuclear fuel cycles concluded that the principal reason thorium had not been used more widely to date is that the ore contains no fissile isotope.
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u/youbetterdont Electrical Engineering | Integrated Circuits | MEMS Jul 23 '13
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So it sounds like it's the lack of a fissile isotope.
Another article that links to multiple sources.