Joy has been an actress for 5 years and had multiple OSTs and variety shows. She has established her individuality before her solo. Notice how among RV members on brand reputations ranking, she's always the one who ranks higher. Plus her song is more GP friendly.
Wendy as being the main vocalist of the group had a lot of OSTs plus MC gigs. And GP likes ballads too.
Seulgi on the other hand. As the group's lead vocalist... She has always been in the group and subunits. SM went wrong in this because I think they should have pushed her more like the others on solo activities. She didn't established her individuality before her solo. Or maybe this solo has been so delayed, that the gp wasn't interested anymore?