Mostly because:
Lady Gaga
-most searched female in the world
-best-selling album this year of "the girls"
-domination that extends even beyond Rihanna's into the Asian world
-made more than twice as much money as Rihanna last year
-by far the largest would tour of the current girls
-the most highly publicized single and album launch in modern history
-the most powerful social media presence in the world
-the best magazine-selling presence in the world