Yoona and Suzy made it to this years isac and the ones before that as well. They were often named the most beautiful idols often surpassing current 3rd gen visuals. I knew about yoona from there whereas I didn't know about taeyeon until I joined akp.
Same with the gp. Yoona had a lot of cfs and dramas so she was in the public eye more. Whereas taeyeon focussed on her music. I've heard even before yoona became an idol she had cf's. She's like the girl people wanna take home to meet their mum.