Because one is Katy's signature song and the other is the video of a song that even go top 50 and the video is not that good for Gaga's standards.
Not that hard to understand.
But if we're gonna ask stupid questions, then why Katy have yet to outsell The Fame (Monster)? Why Gaga's lowest score on Metacritic with a studio album is equal to Katy's biggest score? Why Gaga is more respected as an artist by the Grammys than Katy?