Because out of the twin, the one that would made a sadistically devil/demon is Not Rin but Yukio. With their personalities differences, Rin would make a good devil but with Rin’s Brain, he would be a great demon. Do, when deciding who gets to top, it was more of, who would loook more sexy. Yukio’s too composed for any kind of sexiness expression while Rin has a natural face that changes naturally.
I agree with this (๑•ㅂ•)و✧
And another point is that Yukio is taller haha.
Naze rin wa toppu ni naranai nodesu ka? Naze Yukio ga soko o utsu koto ga dekinai nodesu ka? (╯°Д °)╯╧╧