The fuel in US Naval nuclear reactors is enriched to a much higher percentage than civilian reactors due to size and longevity considerations. It has to fit the ship and refuels take months/years. A ship undergoing a refuel isn't a ship you can use.

In a civilian plant, you can have multiple reactors and refuel them on a rotating schedule to avoid downtime, having a larger reactor vessel isn't a problem, and all of that is also going to be less expensive - which is a huge factor.