Here in Hetalia, I blame the fact that the author didn't add Mexico. Only one reference in the anime, and another reference in the CD drama. One of these days, someone needs to go to Hetalia panels while cosplaying Mexico (Nadiezda design, being more recognizable), so that Mexico awareness will spread beyond the American continent.
One of the first things I learn in California elementary school history, is the Cortez invasion first, only then they go into US history. Because California remembers former parent/boss Mexico being the first of future North American countries to be colonized by Europe.