No-one "decides" it, and it's not (entirely) invented by the media. Fashion designers come up with different stuff each season, shamelessly copying each other all the while, and fashion journalists scrabble to define what "trends" are emerging to justify their own existence. Then people who want to appear trendy and show off their disposable income buy whatever the journalists tell them to (or, to be slightly less cynical, whatever they like the look of in the fashion supplements).
You get fashions in every creative industry from music to cinema to advertising to painting, it's just particularly noticeable in the clothing industry because it's so very pointless and self-justifying and because the seasonal release model makes it look official.
Fashion is crass and exploitative. Media rubbish.
They have to constantly change what's in style to have an income. Funny how one decade something is so attractive-then designers decide it's ugly (forgetting that they're the ones who come up with it) and change to something else--which will be ugly in another decade. Give me my jeans and tees.
The group of business people who what to make lots and lots of money.