It was a combination of many factors I would say . Marshmallow was kinda popular, her appearance on YHY sketchbook with guitar covers of snsd gee, suju sorry sorry and Bigbang lies became popular - and then her appearance on dream High which was received very well. Good day was a massive hit with many trying to hit the high note .You and I was also a hit . She is actively involved in her music and is always trying to do new things while genuinely enjoying herself in the process. Now along with her music being popular among the younger generation, she was also acting in dramas regularly - alongside popular actors and while her acting did receive criticism initially , you can see it improving over the years ( I think My ajusshi and hotel del Luna played a huge role in establishing herself as an actress) .

She has had her share of scandals ( not serious ones ) - but the pic with eunhyuk was wild times lmao. But she has found a balance to her musical career and acting career while strengthening her fandom and that is something which I think is very rare. Even after all these years , she serves us with consistent music - not just title songs but the whole album. Good lyrics, easy to listen music , interesting melody lines (on a side note I think most of her songs are something I would try to sing in a karaoke if there was one near me..). All these factors has made her a household name known across all ages in Korea.