No, it was just easiest to get running. You can make natural uranium reactor with no enrichment of fuel needed, even - it's how Manhattan Project could get fissile material in the first place.

Thorium was actually more of military-only solution than Uranium->Plutonium chain, as unlike Uranium it doesn't have obvious "easy" energy use path, but early on it was researched as mix of uranium and thorium fuels due to uranium availability concerns.