It's nothing against any of their offerings, but each of them are really derived from cultural roots of the Spanish Empire (Spain being obvious, and Mexico and Brazil were both colonies for lengthy periods of time). It's likely just my prior experiences with their food/beverages/etc., but I've always tended to view them as offshoots from a similar base.
I'd likely find the same issue if they decided to put a Scotland and Ireland pavilion into World Showcase. There's nothing wrong with any of them, but I'd prefer to see something new and not so similar.