The answer is quiet simple: it would ruin the lucrative business of the present energy supplying corporations, as in the oil, gas, coal and nuclear industry.
Talking about the nuclear industry... there lies perhaps the most important point why Thorium reactors are not being used: they dont generate Plutonium that can be used in making nuclear weapons!

Here a short wrap up of what Thorium is, and its potential to change the energy supply on Earth: