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.

It's so easy, it even occurred naturally in past.

https://en.wikipedia.org/wiki/Natural_nuclear_fission_reacto...

Thorium for military kind of sucks. If you want to build a substainable long term program you dont want U-232 around.

The only real advantage of Thorium is that you can use it in a thermal breeder. If you are not building a thermal breeder using thorim is mostly pointless.