I know you all will come here and say "but I fInD ThEm bEaUtIful", it's no secret that their looks wasn't popular in Korea or for ifans.
Poor girls got memed so much in both Korea and out for their looks. Everytime there was a fan war people digged out these sistar screen caps.
So on a general term their looks wasn't popular with people despite their songs being great and smashing.
I can't think of any other reason why they had so little fandom and sale power.