Nuclear powered commercial ships were tries, for example NS Savannah. They ended up getting barred from a lot of ports. The international shipping industry is extremely cost conscious, and that includes crew and maintenance. You need highly trained crew to safely run a nuclear reactor, which means high crew costs. Maintenance is also extremely important. If a Diesel engine breaks down it is a mild annoyance. If a nuclear reactor breaks down it is a serious problem.

Incidentally steam turbo-electric drive was used in ships in the past. Several US battleships and aircraft carriers of the 1920s era used it, for example. Geared turbine are generally cheaper and more reliable, though, but modern Diesel engines are even cheaper.