^^ I thought it was because Japan's love Minnie so it would be greater to market them with Minnie as the poster boy. And they did say (jokingly) that Minnie's their Japan's Secret Weapon
That you point about it I didn't really pay attention to it, but it's the same I guess on SNSD
Seohyun and Yuri got the spotlight but they didn't change the singing line as long as I see them. And I thought they really love Yoong too in Japan.