Why do people hate mainstream music?

People at my school hate mainstream music so much. Everyone is scared to admit they like a "new song" that is in the top 100's. That's because everyone at my school is a hipster. They say mainstream music is terrible.Well it's not all bad. I am someone who listens to alternative rock half the time and sometimes i listen to Pink Floyd and def leopard and Rainbow, and then sometimes i listen to mainstream and it's not all bad! Granted, people like Kesha and Miley are pretty terrible. But listen to stuff like Plain White T's, Taylor Swift, Ingrid Michelson, Colbie Calliet, etc. All these artists are way good! People make umptions to easily. If something is mainstream, they automatically think "hate" before actually listening to it. What do you think? I feel imprisoned into only liking the kind of music they like.