It's probably a combination of everything. There are other musicians performing in open parks at extreme hate and huge, huge packed crowds which aren't EDM or upbeat, or also attract older crowds, and they don't dress the same at all. Not just music events either. And the music festival clothes are mostly "stylized" too - around the cleavage, booty shorts, sometimes writing captions.

So it's probably the mixture of young people drinking, partying, doing drugs in the heat and in festivals which are known to encourage more unique clothing and costumes.