Maybe a lot of it is cultural and religious differences? I'm assuming by "Asian" you mean Pakastanis, Indians etc - I exclude Orientals here. Like, a lot of asians are very asian and not very westernised - so there will be cultural issues if he dated a white girl, family disapproval etc, I imagine. Asian parents don't tend to like their kids dating white people, is the impression I have. There's also the sterotype that asian guys are macho, which could put some girls off, and other asians such as Muslims, Sikhs, are not allowed to date - this depends on what caste (sp?) they're from I think, as I've met a Muslim girl who've dated her English bf very openly, but I've also got a Muslim good friend who I know is not allowed to date according to Islam. And a lot of asian guys like girls from their own religion, and most Muslims/Hindus/Sikhs tend to be asians as far as I'm aware. Asians nowadays also generally have a bad rep with regards to terrorism etc, and some people believe asian families to be too extreme or congregate too much, stemming from prejudice, of course, but again this has something to do with the rep asians generally have and some parents don't like their kids dating asians. And a lot of asian families don't believe in dating, or they don't want their children to date til after university, which might explain why we don't see many interracial couples in terms of asians and whites. My Hindu friend's still unsure as to whether her mum would approve if she ever dated, and my Sikh friend doesn't believe in dating full-stop. My Muslim friend whom I've mentioned wants to date but her parents won't even let her access the internet or in fact go out to town with her friends, let alone date.
^the above are all speculation by the way, so don't take it as facts. Just people I've met etc as there's a huge asian population around the area where I live.