It's not that Spider-man couldn't be represented as such in an alternate universe. However, he was never really portrayed as such in the comic books. Personally, I don't think it is a race thing either. That's like having a non American play Captain America. Or having a non caucasian European play Bond. It just is not what people can imagine or envision. It has nothing to do with prejudice. But rather what the character has been defined as (already) within the industry for so long.