Thorium has significantly less delayed neutrons [1], which means that the reactor is closer to prompt criticality. In addition, browsing the relevant parts of wikipedia [2], there seem to be some problem with the neutron economy. And to start an reactor you need to breed some U 233 from Thorium first ( Which will alter the reaction of the reactor to a given neutron flux). Since the neutron flux in the reactor is the main variable which controls the power output it seems, that a Thorium reactor is significantly harder to control than a Uranium one.

[1] .6 delayed neutrons per 100 fissions for U233 compared to 1.6 for U235. http://www.reak.bme.hu/Wigner_Course/WignerManuals/Budapest/...

[2] Especially https://en.wikipedia.org/wiki/Liquid_fluoride_thorium_reacto...