Rihannaaaaaaaaaa MAJAH MAJAH BRRAP. Pitbull... Adele slaying with only 2 peaked singles and another single just getting started. Gaga still selling great.
Because she didn't release any new music or featurings this year.
Even if we were to look at the time period from California Gurls first dropping last year to the equivalent week this year, she wouldn't be #1. There is no year-span in history where she was #1 on this chart.