So "Asian" apparently isn't an acceptable answer, but "wigga" is? Huh?
It's probably just on the much less curated list of words that are accepted as guesses but not solutions. I highly doubt it's in the solutions list.
Given that Asian is always capitalized, it understandably never made it on either list. Proper nouns/adjectives are easy to remove.
Whether Asian is a proper noun or an adjective can only really be determined by the context. Since there is no context here, perhaps proper noun is assumed. If Asian had been allowed, perhaps there would be a topic about why a proper noun was allowed, so the developer is in a no win situation.
Also, may just be coincidence, but wigga appears in the Scrabble dictionary and Asian does not.