Because Onika got this one right. She took a great hook and added something great, making it her own. Even Sir Mix-A-Lot had to shout.... well, OK, he didn't shout, but he does love the song. (And the video - he is a documented ass-man, after all).
And I like it, too. It's hummable, and let's face it, Nicki Minaj has got skills. She understands hip-hop, both musically and culturally, like few other artists, male or female, do, and it shows. I hope you enjoy this as much as I do.