Capital costs and round-trip efficiency are the primary reasons. Even at constant utilization, electrolyzers are expensive enough that it's hard to replace fossil-fuel generated hydrogen at the moment.

Once you 2x-10x the cost of electrolyzer capital by only using it rarely, more generation and throwing away the excess electricity often makes the most sense.

Nuclear ships are similarly super super expensive. The only reason we build them at all are for their unique and wonderful operational capabilities, as in not needing to surface or refuel. Using nuclear ships for power would so expensive that we may need to up our GDPs 10x before such wasteful use makes sense. (Though I'm hoping we do reach such future luxurious lifestyles!)