Okay, call me arrogant and say I don't understand life but seriously, I hate boy bands! Why are so many people into Panic!At The Disco or Fallout Boy or My Chemical Romance? I'm a fan of music like Chenille Sisters or Indigo Girls, their songs are brilliant and motivating and such, but why all boy bands? Seriously, it's a little old fashioned. And before you go ranting about how female bands have taken the lead or that crap, take note that all of history has been pushing women away from center stage. Up until way into the Renaissance, men always played women's parts in plays, and fat chance a woman would get a chance to do any form of entertainment. Okay rant over now.
I'm not sure, but this phenomenon is not necessarily modern, it's been a large thing since the 20th century if you include Beatlemania.
A lot of the boy bands are meant appeal to teen and pre teen girls, especially with the sense of a romantic ideal towards the band members. You may not necessarily find that with girl bands and teen/pre teen boys - I don't know if it has anything to do with how widespread/established in the market girl bands are compared to boy bands, but it might play a part.
But, girl bands are also popular beyond the West, especially in East Asia. The first few paragraphs of this article explain how the main market for young girl bands is middle aged men in Japan. Also, a lot of East Asian bands, like K-Pop ones, are styled like reality shows where the band member's lives are constantly followed. So this may raise the appeal for them, like how the Kardashian-Jenner family's show Keeping up with the Kardashians has been very successful and makes the family more noticeable.
I mean I'm not like hardcore on boy bands, but I just really love their music.
