I rather suspect that one of the principle reasons is the creating and maintaining of the physical security required for such sites. Imagine a country scattered with mini nuclear bombs situated in the middle of nowhere and the cohorts of undesirables coming into the country nearly every day - pc term is 'Bad Actors' who only have to get lucky once- and then imagine the logistics of keeping each site heavily guarded. They have enough problems at the likes of Sellafield and other sites..
You can make a "dirty bomb" though - conventional explosives scattering the not only radioactive but usually very toxic fissile material far and wide...
Indeed, but you only have to violently destroy the reactor to contaminate a wide area. A ship at sea, especially a military one, is a wholly different entity.
