Because Rihanna just has that starpower and Katy doesn't. Also Europe doesn't use Katy almost at all.

Because her music is better/she's better looking/her voice is better.

Originally posted by monsterx by the GP Katy Perry > Rihanna

Originally posted by PopFan She has 6 albums

Originally posted by Siya Rihanna is just better. No more explanation needed.

Originally posted by crumprKID Their image. Katy's is very light and fun while Rihanna is marketed as 'edgy' and 'badass'.

Originally posted by VPM Rih's a slightly better artist. At least better than Katy. More unconventional.

Originally posted by monsterx On ATRL, by the GP Katy Perry > Rihanna I don't know why..

I think they're the same.
Both making seemlessy catchy music and hits.
They both have the same amount of substance.
They resemble each other very well IMO.
Though Rihanna seems to have a slight edge over Katy.