Generic ninja has only two lead-ins: the fading popularity of Power Rangers [albeit not exactly ninja, but the martial arts influence is there] and the slight incline of Naruto fandom due to Cartoon Network exposure. But kids can buy Power Ranger uniforms while Naruto costumes are non-exsistant for those outside the cosplay society.
Meanwhile, pirates have Johnny Depp literally at the helm. And with PotC2 shattering box office records, did ninjas even have a chance?

Oddly enough, a magazine article I'd read earlier today listed pirates as the top costume to be expected this year with princess [of any flavor] down at #6. Just below celeb lookalikes.