Why build a ship for limited sea conditions?

Why build a passenger ship that is not intended to cross the Atlantic in all conditions?

Because many modern cruise ships are optimized as Caribbean leisure destinations, not as all-weather transatlantic liners.

Many modern cruise ships are built primarily as very large Caribbean resort platforms. Their commercial purpose centers on entertainment and accommodation along selected routes and in selected seasons. They are not designed for dependable ocean passage in every condition. A peaceful transit in favorable weather does not establish fitness for wartime or severe seas, and sheer size does not prove universal seakeeping ability.