They're new and experimental
Hi SDF
Two things mean they're under-invested in.
(1) Uranium mining industry would be gutted by reactors that don't need anymore ore mined for decades. There's enough 'waste' to last for decades.
(2) Water-cooled enriched uranium reactors are 'good enough' and building them has become commercially viable.
Both mean that Governments have been reluctant to pursue thorium and fast reactors with R & D funding - except in places like India with scads of thorium and not a lot of uranium.
Adam