The first iteration of any new technology has problems. Assuming, as the article claimed, that the two technologies were both considered viable options, the considerations seem to have been:
Thorium -- Comparatively cheap, more controllable (assuming significant materials science development solves corrosion issues), comparatively minor waste containment issues
Uranium -- Much more expensive, difficult to control the reaction, major waste containment issues, produces weapons-grade output
With today's world-view, everything about Uranium reads as a negative. But during the height of the cold war, when we subjected our children to duck-and-cover drills, weponization was seen as a good thing ... as long as it was us doing it and not them.
If all the money spent on current nuclear technology had instead been spent on thorium, you don't think we'd have improved the efficiency compared to the French example?