Well first of all when you say "Asians" you also refer to Indians and other south Asians. But interracial relationships with blacks are rare here as well. I think both East and Southern Asians have the same reasons.
Cultural clash, my mum wouldn't approve of a black girl. (Not like i'm straight anyway

)Not black, white or East Asian. She's not racist, but she says how will she be able to have a good relationship with the girl's side of the family, and being honest it's true. With her broken English, there's no way she'll naturally get along with a white/black family as she will with another Sri Lankan/Indian family. But that's her own fault.
Also
Asian parents are extremely controlling. I reckon most 2nd generation Asians have no problem with interracial dating, but they wont due to the ""My dad/Mum will kill me if they find out".
Also
negative stereotypes. Not only do all old fashioned Asians look down upon darker skin, even if its just beauty wise. They usually generalise a LOT. They think a lot of blacks are ghetto, and the rest are well mannered, conservative, church going Africans. You are in one box or the other. It's something in general asians do a lot.