Due to lack of fitness
Due to fashion or culture
Due to complex
Firm fitt and healthy breasts do not need the support of any artificial thing like bra to prominent them, In west people hate those females who artificially shows our breasts through bra's and girls always stay without bra because they are confident on our figure and like to live in our innocent natural curves
In most asian/middle eastern countries Women have less difference between her waist and breasts size, Without bra its difficult for local women to show our curves naturally and she always use bra to look feminine, 99.9% women have this problem and in complex to prominent our breasts through an artificial way with brassiers
I got this idea personally and through after viewing Tv Movies and Media