Just a practical way of harnessing its power.
Thorium power-plants need to go a long way it before being commercially interesting. Not because Thorium is expensive or hard to use, but because we don't really have a easy way to tap on its power — yet.
People are driven by necessities. If coal and oil (the "easy" power sources) are getting rarer and more expensive, people will turn to other, "harder to use" energy sources, like nuclear power, hydroelectric or wind-based.
However, as you use something, you learn its limitations. This provides a good base to future research, which in turn makes it easier to use as new, more efficient stuff is discovered about that something. It was the same with computers, with the Architecture change — people didn't swap to x64 right away because it was easier to keep using x86, even if x64 was way better. Nowadays, you only find x86 computers on some really outdated offices and on a few grandfather's houses. Heck, it's HARD to find x86 machines anymore!
So, yeah. It's mostly time. Once you find a way to make use of Thorium, just keep up researching and making it easier to use, and eventually people will embrace it.
