Total stab in the dark but i suspect it might come down to basic physics of lift vs weight. Any kind of nuclear powerplant would like be very heavy as would the ancillary materials needed eg cooling systems and insulation to shield the cabin so it's perhaps not economically viable in the short term to be considering it??
Although Iran may yet surprise us all (sorry, couldn't resist.)
This purely a half baked guess though so please don't tear me apart.