What might account for so little overlap in the land areas of the Mayas and the Aztecs?

Well, honey, it's simple math. The Mayas were chilling in the Yucatán Peninsula, while the Aztecs were throwing parties in the Valley of Mexico. It's like comparing Apples and Oranges, they were just doing their own thing in different neighborhoods. So, of course, there's gonna be little overlap in their land areas.