1. Relatively low maintenance. You can leave a cat alone all day and they generally take care of themselves.
2. Realtively quiet. Unlike noisy dogs, cat's don't bug the neighbors.
3. Litter box. Enough said.
4. They don't need to go for walks.
5. Generally speaking, they don't scare people.
6. Cute factor.
7. Good size for petting. I hear it lowers one's blood pressure to pet a cat.
8. Cheap entertainment. Wiggle a string in front of a cat and it goes nuts.
9. They live longer than dogs.
10. Purring and affection.