I think it is due to a fear of appearing different. The basic wardrobe is easy to follow and allows for variations on a general style. Each item of clothing can be varied e.g. nike or adidas trainers, within a comforting framework. It allows young people to be different at the same time as being conformist e.g I've got rarer and more expensive trainers then you. Anyone who tries to dress differently (e.g. Goths) are derided by the majority, thus reinforcing the need to stay within the basic framework of trainers, trackie bottoms, polo shirt and baseball cap.