The story I've heard is "It's almost as safe and easy as fusion will be, but you can't use it to make nuclear weapons so no governments investing in nuclear were interested".
This doesn't explain though: Why not both? Why not now?
Even if you decide that investing in uranium/plutonium is a must, you can still choose thorium over coal/gas/renewables, going for both nuclear paths. It just doesn't make sense, unless thorium isn't as good as people say.