The thing is, ninja's have been around since ages ago but kids wouldn't know about them till they are around 13 because some ninja manga's are kinda rude whereas pirates are new but its ok for children to know about them because Disney mussed up their image.
Being a pirate isn't that bad I mean yeah it's breaking a ton of laws and stuff. And you're want for a ton of crimes. But it's still fun. Sword Fighting Looking for gold and sailing, And blowing up other bad ships that come your way.
also, pirates can be easily shot. ninja's know that speed is the essence and that they need to dodge that bullet unlike pirates who don't see the hit coming.
i hate these kind of decisions, pirates of the Caribbean have won me over because of all the wisecracks and funny jokes, but ninjas for the awesome silentness and stealth.