(To be more specific, it's the absence of nuclides with mass number greater than 238 in Thorium waste that makes it less problematic than Uranium waste. These nuclides dominate the long-time, i.e. millenium, radiation profile of the waste. The short-time, i.e. decade/century, radiation profile of the waste is dominated by lower-mass-number fission products, abundant in both types of waste)
Reprocessing plants are extraordinarily expensive to build and operate, more so even than reactors themselves.