The 9-piece girl group is SNSD/Girls' Generation and the sub-group with the "Twinkle" song is named TaeTiSeo. The main group releases music in both Japan & Korea, that's why they can fall under both genres (JPop, and KPop)
Because the 6 other girls are busy with their own endeavors, reality shows, dramas, musicals etc..
Scheduling conflicts probably would have made a full group comeback hard. So why not a subunit? And to be honest it's about time. Too bad it's not the subunit I wanted.