What a bummer. Would travel insurance cover this? Did you have any?
I'm not sure about the other lines but I believe most use a contractor for porter services. At a pier with many ships of different lines I believe the porters just move from one dock to the next, servicing any ship that is embarking. So blaming Carnival, or Princess, or anyone for lost luggage is probably not the correct thing to do.
However, Customer Service is line specific. So if your luggage was lost on another line you might have received better compensation through a different cruiseline. Maybe not though, I just have never had a problem to find out.