Basically because they have been conditioned to not talk about it. When society in India is still hesitant to allow sex education in schools, talk about mensural cycle, limit interaction between young men and women unless they are married... discussion with each other (girl to girl) also become difficult..
I don't believe it has anything to do with social conditioning. I am a woman and I can talk about orgasm because I experience it. Very few women are responsive so they never discover orgasm. They think that they are supposed to orgasm from stimulation provided by a man. They confuse emotional responses for the arousal and orgasm men talk about. Any one who experiences orgasm can explain how they got there - the precise anatomy, the erotic turn-ons that cause mental arousal & the stimulation technique. Orgasm is a basic, instinctive and repeatable experience. Sex education is about much more than reproduction. It involves explaining how orgasm is achieved by anyone regardless of their sex or orientation.