I think you really hit the nail on the head there - people don't consider what having Khan be non-white would actually mean. What it would look like or reinforce. It's understandable in a way; it's practically always a really bad idea and racist to change ethnicity from anything to western/white, and it's easy to rush in without considering all the specifics of every character. Though I think that if one wants to write meta on him, one really should take a thorough look at the character, heh.