Seeing it from this perspective, maybe I now see it: the residual radioactivity inside the reactor may be strong enough to mess with the electronics of a robot you're sending inside. Maybe there are challenges in getting enough shielding while keeping the robot reasonably sized and maneuverable.
A major challenge is getting the robot into the place you need it, along with other objects it needs like replacement parts, in between the other infrastructure like coils and cooling blankets. You mustn't damage the delicate reactor structures, you can't have rails and so on always in the reactor vessel, the vessel has to be vacuum tight, and most of all you must not allow the robot to fail and be unextractable, because humans can't go in an pull it out.
ITER seems to have a project called the Agile Robot Transporter which you can see some renders of that demonstrate how fiddly this problem is.
