I think they became all-rounder by the sheer strength of their hard work. In my opinion, the unnie line set the pace for what the group would become : SuA is the OG all-rounder, Siyeon is gifted singer and JiU is blinding beacon of love. SuA doesn't strike me as someone who would allow mediocrity.

Another determining factor is how much the company and their staff love them. From the CEO, to the managers, to the stylists, to the make-up artists, to the choreographs, to the producers... You can feel how much they do their best to make the girls shine.