the initial research being for weapons is probably part of it, though uranium-233 (from thorium) was used during a 1955 nuclear bomb test. [1]
thorium reactors are also "breeder" reactors. they initially require an external source of neutrons to start the reaction, then the thorium breeds into uranium-233 and the reaction can become self-sustaining. so thorium by itself is actually an impossible place to start.
Good point; compare to the first 5 US reactors, the Chicago Pile and the plutonium breeding Oak Ridge X-10, and Hanford B, D and F reactors. All graphite moderated (very pure graphite, BTW, something the Nazi's couldn't get), all using natural, as in unenriched uranium (at least initially). That was obviously handy for bootstrapping prior to enough Highly Enriched Uranium (HUE, mostly U-235) being available from Oak Ridge's separation plants, which I gather the first major use of was for the Little Boy gun assembly bomb.
That initial charge can be the Trans-Uranics (TRUs) from spent nuclear fuel. That way we can transition into the Thorium Age while cleaning up after the Uranium Flirtation! ;)
thorium reactors are also "breeder" reactors. they initially require an external source of neutrons to start the reaction, then the thorium breeds into uranium-233 and the reaction can become self-sustaining. so thorium by itself is actually an impossible place to start.
[1] http://en.wikipedia.org/wiki/Uranium-233