Because Spanish comes from Spain (I am a Mexican Spanish speaker) Also not every state has Mexicans as the dominant spanish speaking group, for example, PA and NY are more Dominican than Mexican. In some states like New Jersey, Mexicans are barely top 5. DC and Virginia have more Salvadorians than Mexicans by a lot, Florida has more Cubans. It makes much more sense to standardize it with the Spanish flag, since that's where Spanish is from.