Ignoring any problems with pollution controls, I think that the biggest thing to look at is the difference between the maintenance and manning costs of a steam plant vs. a diesel plant.
Snipe 70E has already pointed out that there are many diesel plants that are operated without anyone in the engineroom - the computer monitors the operation of the diesel, and flags any unusual conditions for the bridge watch to have investigated. I suspect this means that a ship running on a diesel plant can get away with having one, or maybe two, trained diesel mechanics, to handle any minor problems might come up during a voyage. (Major problems are going to be beyond the means of shipboard repair, anyways.) Which makes for a fairly cheap payroll associated with propulsion. Maintenance costs for a diesel plant are similarly relatively cheap: there is some wear and tear on the moving parts, but having a good lube oil system going keeps the moving parts lubricated, cleaned, and cooled. Some of the contaminants in the fuel can be pretty active, but again AIUI most fuel oil these days is refined to minimize the sulfur content, which was the big corrosive contaminant I’d heard about back in the day.
With a steam plant your manning goes up considerably. For a quick comparison of how a steam plant can multiply manning considerations, consider two old USN cruiser classes: USS Ticonderoga class, which uses gas turbine power plants vs. USS Virginia class, which used nuclear fired steam plants. I can’t claim that they are exact duplicates except for their power plants, but they’re a pretty good match in size, mission, and armament. The Ticos had a crew (per Wikipedia) of under 400 men; the Virginias had a crew of just under 600 men. That’s a 50% increase. I’m not going to claim that all of those extra warm bodies were in the engineering department, but I believe that most of them were either there, or added to other departments to support the extra crew loading that the expanded engineering department required. And that’s comparing to gas turbine plants, which are more complex and require closer monitoring than a civilian diesel does.
Aboard Virginia we had a watch team any time the steam plant was in operation (and this is ignoring anyone whose duty was solely for reactor-only operations) that numbered about a dozen. Now you can’t make a complete comparison between civilian manning and naval manning: Naval plants run with more bodies because that’s the best way to deal with unforeseen situations. So, cut that in half for a civilian plant, and we’re still talking about having five or six people on duty at all times in your steam plant. To cover three shifts, that means you’ve replaced that single or two diesel mechanics with eighteen people, plus at least one extra cook, expanded crew quarters, greater food costs, and other hidden increases, just to accommodate your larger crew. Look at the M/V Maersk Alabama (a diesel merchant ship I could name off the top of my head) as standard - she has a total crew of 21. You’re nearly doubling your crew size to switch to steam. That’s going to really cause a jump in overhead costs.
Then there’s maintenance. Steam is a nasty bitch to deal with. It cuts metal, as well as other substances. It also means that your surfaces are all going to be covered in moisture, which is a great medium to promote corrosion. Even with a properly conditioned steam plant corrosion and wear are constant battles, requiring a lot more maintenance than a diesel plant does. Let even a little contamination get into your steam plant and the corrosion rates go through the roof. Which leads to the whole safety issue: steam kills. Diesel plants can be deadly, no question about that, but you don’t hear about diesel plants blowing up and cooking their whole crews. Steam explosions are a lot more common than any one wants to think about. I was active duty when the Iwo Jima LPH-2 cooked a whole watch standing crew. On the one hand, you can say that the accident was the result of a botched valve replacement, but the potential for that sort of disaster was there with the steam plant which required the more complex and more frequent maintenance than a similar diesel plant would have needed.
Also touching upon maintenance is that fueling a coal fired boiler plant is going to be more complex than fueling an oil-based boiler plant, let alone a oil-based diesel plant. At the very least you’ll have to have hoppers, conveyor systems, and means to control the coal from shifting at the ship rolls in heavy seas, and to balance the load as coal is burned. In the old days this was often done manually, but that would even further jump up the manning requirements mentioned above.
The old coal fired steam liners, freighters and tramps have a rosy, and romantic, image these days. You can get me waxing nostalgic about them very easily. But compared to a modern diesel plant they were very expensive to operate. And that, not any environmental regulation, is going to be what keeps them in the past.