It’s all fun and games until your (insert vehicle here) crashes , or has a fire, or suffers a mishap, or reaches its destination and explodes as designed, and apart from all the normal problems you have with that, you also now have to contend with a few kilos of fizzed up nuclear fuel and some hot reaction by-products spread all over the place. You also have to contend with the neutron activation of the air passing through your nuclear ramjet, which makes it briefly radioactive, which is fine for a cruise missile that you intend to blow up in a few hours anyway, not so fine for regular transport routes.

Nuclear powered vehicles have some inherent risks with pain-in-the-ass consequences, and if we scale those small per-vehicle risks up across a worldwide fleet we’d see accidents involving them as often as we are aircraft crashes, and that’s not great.