From what I've heard, it's because we've now got decades of experience engineering uranium fission reactors, during which we've learned tons about the practical

From what I've heard, it's because we've now got decades of experience engineering uranium fission reactors, during which we've learned tons about the practical engineering challenges involved. While thorium looks great on paper, engineers expect we'll need decades of working with it before a similar body of expertise is available, during which accidents may occur that we know how to avoid with uranium, etc. In other words, without uranium being scarce and expensive, and with thorium being lumped in the same "nuclear" bucket (with all the political hurdles that implies), it doesn't make much economic sense to start over with thorium.