Because along with a steady source of Pu238, you also get the benefit of a safer, smaller nuclear reactor using a fuel that is significantly more common and less weaponizable than uranium/plutonium.
(The thought being that you can then only allow one unit to be in transit from storage to a reactor at any given time, pretty much entirely mitigating the risk of someone gathering enough of it to make a bomb)
Economic feasibility: http://www.thoriumenergyalliance.com/downloads/plutonium-238...