Nuclear reactors are cost-efficient on large scale, and quite expensive on smaller scale; if you have nuclear reactor that makes 100 times less power, you don't get 100 times less complexity/risks/maintenance/manpower/etc.

Nuclear power makes sense for if you need a lot of power for a ship and/or very long range without refueling, so aircraft carriers, missile subs and (barely) long-range icebreakers are a good fit; but diesel is not a big problem for a ship that's designed to be fuel efficient and is on a route where it can refuel frequently.