If I recall, the role of Simon was open to Asian actors. Joss had cast Summer Glau as River right from the beginning - that wasn't negotiable - and since she has a slightly Asian-ish look to her face, it was possible that her brother would be of Asian ethnicity. But then whatshisface auditioned, and the decision was made.
I guess that doesn't explain why none
of the cast were Asian, though. I suspect Joss just had people he wanted already in mind for the big roles long before it got to the actual casting stage.