I've been wondering about this for a long time and there's really just no literature on the subject. Some thoughts --

  • the visual effects and color schemes are somehow crucial to their success, expressing fertility or feelings of attraction

  • the group-size (5-8) is the ideal prehistoric warrior "tribe" size, increasing the attraction more than the sum of each individual member

  • the popularity is manufactured by the gigantic corporations that manufactured the BTS group and writes their songs. After it takes off it's reinforced by the corporation, yet the young female audience doesn't know this and instead think their sex appeal and popularity are organic (in turn, adding to their sex appeal and popularity, because they are previously confirmed by other females)

  • the novelty of liking an exotic band attracts young women who are isolated or rejected from popular American culture, or who just want to be different