Why do so MANY girls in my school only wear Abercrombie, Hollister, etc..?

I'm in the 6th grade and almost every girl in my school only wear Abercrombie and Hollister. Like, the only shirts they wear is a plain tee with either Abercrombie or Hollister printed on the shirt. It pisses me off that they make fun of me for wearing clothes from Forever 21, H&M, Zara, J Crew, Levi's, Banana Republic..... they assume that if you don't wear Abercrombie or Hollister is that you're a poor.... Even my cousin thinks the SAME way, she's like, "If I we're rich, i'd only be wearing Abercrombie" then yesterday, she was like, "Why do you get clothes from Zara??" then she gives me these weird looks. What happened to expressing yourself with your clothing??