Well the first conclusion to jump to would be racism, but I don't think that's usually right (of course sometimes it is, as all groups have some people who are sometimes racist).
For me, the Chinese community I grew up with was very tightly knit. For example, my mother has lived for 30 years in the US now, and she has many "friends" at work who are not Chinese, but outside of work, she never spends time with these people; her real social circle outside of work consists of my dad (who is also Chinese), my sister and I (also Chinese, duh), and three friends who are all Chinese women.
This is really common, and part of the reason for it is that Chinese culture is just so different from American culture that many immigrants just never really adapt and stay nestled within the Chinese community. I've been to China many times, and even for me the culture shock is pretty huge.
So I'd say it's less racism and more just general xenophobia; most of my Chinese friends have formed a tight knit community that is kind of unwelcoming to outsiders of any kind, be they black or white or indian. Luckily for me my parents don't seem to mind the idea of me dating non-Chinese and non-Asian guys; they're okay with me exploring the outside world, they're just personally more comfortable staying inside the Chinese bubble (that's actually what we call it), and I think many girls, even younger ones like me, feel the same.