Members who leave SM can't have music careers in Korea.
Tiffany only did music while in the US, when she went back to Korea, she stopped and started acting in dramas and musicals.
Seohyun is yet to release music at all.
Sooyoung released a ballad and another song that she didn't promote all. They didn't add to her music career. She's only doing music in Japan!
Which means, one of the terms in the contracts with SM is that they can't have solo music careers in Korea if they are to remain Snsd which is sad.