It’s probably as simple as we already have something successful. Why spend time and effort overcoming the challenges to create new reactor technology with many of the same benefits and shortcomings as we already have?

I know the arguments for thorium and can see that being a huge benefit to places without a mature nuclear industry and without developed fuel sources.

Sure it would be somewhat better for us as well, but the biggest limitations will be the same. You’re still impeded by fears of radioactivity even if it is less. You still have radioactive waste to handle even if it’s less and less long lasting. You still have legal and regulatory challenges driving costs and timelines through the roof. Thorium hasn’t won the war of public perception, so is no better in the things that actually impede its use