Because kpop fill the void of girlband and boyband that hasn't been produced for the long time in west. Ifans want parasocial relationship with these kind of groups. Idols are specifically made for these kind of listeners.
But Kpop soloist is an artist, unlike idols which basically product of their company, they produce music and that's the only one they sell. They don't sell image, persona just like idols do. If you want to attach to the song, you need to understand the lyrics. Most fans don't understand Korean. it is difficult to build emotional attachment to the song and why kpop soloist will not be as famous as kpop group.