Carnival has never put many ships in Europe - usually just new ones comong out of the shipyard and another big reason is that NCL and Royal Caribbean have both flooded the European market, not to mention Costa and MSC, for "affordable" cruises on mainstream cruise ships.
For me, you want to see Europe on a nicer ship; Holland America, Princess or even better; Oceania, Azamara, or a luxury ship.
The reason is that you want to see the sights - not hang out on the ship. You don't need a ship with Broadway shows and 5 water slides in Europe, you need a comfy bed, good food and a quick breakfast.
Also - Costa ships are exact copies of Carnivals (except the decor) so "Carnival" already has an entire fleet in Europe. If Europeans want to go on "carnival" they book Costa. Which explains Heald's comment that Carnival's audience is "Americans"
I can just imagine a European going on a Carnival ship and seeing that it is identical to the Costa ship he took last year. I mean the stateroom has the exact same furniture. The only difference would be that no one spoke his native language and the food would not be to his liking.
I once took a Carnival ship and happened to have a night scheduled for dinner on a Costa ship in Ft Lauderdale a couple nights later. So I was one night away from carnival before I walked onto the Costa ship. I had literally the identical stateroom.
The decor is completely different, but they have spirit, destiny and conquest-class ships, and soon will have a dream-class.