Logistically how would they manage speciality food carts? The cruise kitchen works at maximum now preparing three meals a day for passengers in the main dining room...including those passengers who have special requests and food allergies. And this is just with the meals already on the menu! Plus you have the speciality restaurants, with their special kitchens and they're working at maximum to prepare for their guests.
Where would these chefs prepare the food for these speciality carts? Which kitchen would they work out of? If you have ever seen or visited a cruise ships kitchen you know every space is used...the chefs are busy as it is with just the prep work for the next meal on the menu.
Sooooo doing speciality food carts may not be a worthwhile project for the cruise line, considering the prep etc.
We go on cruises to experience something new and different anyway...so why the struggle to bring the same brand restaurants on a cruise so you can have the same experience you have at home. There's no fun in that...