Because a significant proportion of South Asian parents have an extremely backwards and medieval mentality when it comes to issues such as dating. A South Asian girl is therefore often placed in a moral dilemma. It is natural and moral to want to respect your parents wishes, however it is also morally wrong to disregard a guy who YOU like simply due to him not being of the same culture/same race as yourself.
I think most young South Asian girls in this country would like the freedom to make their own choices. If having someone who is of the same background as yourself is an essential quality to YOU NOT YOUR PARENTS then I can respect that. If, however, you are more open minded and simply want a good man who you click with and who treats you with the respect and dignity you deserve regardless of his race or background then you should not let the repulsive view that you are obliged to "stick to your own" hold you back if you meet a white or black guy who has qualities you desire. To have the latter mindset requires one a lot of willpower and strength that unfortunately not all South Asian girls possess (e.g. My ex) as the potential of losing their family is too much for them to take.
But some South Asian girls pull through the hostility such as my cousins gf. He is black and she is Gujurati Indian. They kicked her out when they found out about their relationship.He proved to her parents that he wasn't trying to "take her away from her culture" and,in fact, he respected the preservation of her culture so much he learnt Gujurati and learnt how to cook numerous Indian dishes e.g. Pav bhaji. He has promised to pass down Indian cultural traditions as well as his own Caribbean cultural traditions to their children. It took time but her parents finally accepted him. Goes to show that cultural differences can be overcome with a bit of perseverance and strength from both parties.