Ninjas are so boring. They have a monotone outfit, kill people all day and then practice killing people after that.
But Pirates are so dynamic! Sure they kill people but they also look for booty with their telescopic spyglasses, train little parrots to sit on their shoulders, sink ships, wear colourful outfits, sing songs, swab the poop deck, fly flags, set fire to things, wear eyepatches, wear peglegs and hooks, shoot cannons, associate with salty sea wenches, and more!
Its clear that pirates are best. And furthermore - You see more blockland pirates than you do blockland ninjas!