Cost and scalabiltiy with speed.
Nuclear reactors are good at generating a constant output which is useful on vessels expected to operate for long periods without having to dock(like carriers which can have supply support ships come to them), but not as useful when you actually have to get to ports often and stopping to Load/unload.
this sort of reactors are not large enough to manage the cost of having to start and stop them somewhat regularly, and making them large enough for this to not be an issue means they arent really usable on Ships anymore due to their sheer size and weight
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