Muti-talent and multitasking.
Edit: Not that Taeyeon is not successful but compared to IU, there's a difference.
IU, Yoona, and Jessica and other idols do more than 1 thing at a time, successfully.
IU is an idol, actress, and big-time CF model, and she has been on top for a long time. She's not the best vocalist out there but her power in the music industry is insane.
Yoona is an idol, actress, CF queen, model, MC, and variety show star and she only performs better every year. She's kind of a veteran but still very attractive and very bankable. It'll be forever before she loses her position.
Jessica is an entrepreneur, model, author, influencer, fashionista, variety queen, and you-tuber. She's in her own league as a nonactive kpop idol who is very relevant on the beauty and fashion scene. Given her situation, she would be buried somewhere struggling with depression, but no, she's busy killing it on her own.
Is that why idols like BoA, Luna, Wendy, and Taeyeon (to some extent) don't reach the same heights because they are mostly vocalists that only model for a bit or even not?