Same reason most racial minorities are Democrats: because they've been brainwashed by Dems that the GOP is racist. Vietnamese-Americans are an exception to this, and I wouldn't be surprised if there are a number of Chinese-Americans, Korean-Americans, or Cambodian-Americans that are Republican.
Or maybe they are viewed as insane gun-nuts who are illogical, don't believe in evolution and are generally skeptical of science? Also, I don't think Republicans are viewed as racist, but more as xenophobes. Talking about "real America", "small town values" and generally acting like a hick is going to repel Asian voters.