Well, I can't speak for anyone but myself but I guess it's because I didn't realize ancient cities like this existed in the United States. Now that I know they do, I find myself wanting to visit and curious to learn more about the history.
AFAIK most if not all aboriginal communities were hunter-gatherers (unless you include torres strait islanders), not settled agriculturalists, hence didn't have large fixed-place communities (and thus no source material for ruins).
I know a lot of people won't like that, and others will say "But security!" That's fine, but it's my choice and that's why I've chosen that.