Carnival has always touted themselves as the ships for the party crowd. They use to call themselves 'The Fun Ships' and now 'Fun for all and all for fun'.
European cruisers, for the most part, are a bit more reserved and like a little more 'class' in their cruises. This is why the more upscale cruise lines like HAL and Celebrity do so well over there.
And as Paul stated, Europeans prefer Costa over Carnival because even though it's very similar to Carnival and owned by Carnival, it's definitely designed more towards what Europeans prefer. This also explains why Costa is not a real big seller for Americans.
Plus, the average Carnival cruiser, quite frankly, is looking for a cheap cruise. So they're not wanting to spend more for the airfare than the cruise costs. (I'm talking average, not the exception.)
The average cruiser who doesn't mind spending alot for the airfare is simply not interested in Carnival; they're wanting something nicer.
In 11 years, we've sold alot of European cruises, but we've only sold Carnival in Europe maybe an average of 3 or 4 times a year and all to Americans. We have clients all over the world and never once have we sold a Carnival European product to anyone outside of the U.S. It's just not a big seller compared to the others, so I can definitely see why they want to move their ships where they have more of a chance to fill the berths.
But it'll be interesting to see what the do next.