Or impose lots of stricter rules on all ships in ports. Cruise ships will follow them to get the customers; container ships to get at the consumer markets.
The difference is that other ship types don't have an incentive to run their engines once they're docked.
Cruise ships as floating hotels have high power requirements even when they're not underway.
They're also likely to be docked at "the marina", or another downtown or residential area. Container ships will typically dock at transport harbors, which are in industrial areas outside of population centers.
