* All the fun and "not thinking" of an all-inclusive resort (though obviously only if you pay for the drink packages) on land
* Generally cheaper than all inclusive resorts on mainland USA (I'm not as familiar with Europe)
* Competitive on pricing with all inclusive resorts in the Caribbean/Mexico
* Get to skip out on flight to the Caribbean/Mexico
* Get to skip out out of the overt semi-colonial feel of like... a Caribbean/Mexican all inclusive resort. If nothing else, while the crew (ie the people running the ship) are almost certainly going to be mostly south/south-east asian, the staff (ie the people actually supposed to interact with the passengers) are going to be sufficiently multi-culturally mixed to help make all those thoughts fade away...
And let's be real about most Caribbean/Mexican all-inclusive resorts... they aren't always the cleanest, and most people don't leave them except on tightly planned excursions anyways.