Well, how do you explain the fact that there are still more than 1.5 million people in central Mexico speaking Aztec (Nahuatl) language?
Mexico City - the oldest and largest city in America (established in 1325 as Tenochtitlan) never lost its demographic continuity. Well into last century there were neighborhoods which did not use Spanish language.
Well, I am not an linguist per se; if I were to turn the tables around and ask you why some tribes in Northern Africa are speaking German...what would be your answer!?
Furthermore where are you getting your stats from?
It is inconceivable that a nation of people, even reduced as it was to less than two million, simply disappeared. The very term Mestizo refers to the mixture of Spanish and native blood. Of course the people of Tenochtitlan didn't even call themselves Aztecs, but by whatever name you can be sure their bloodlines carry on.
On top of that, besides Aztecs, there are their stay-behind cousins who didn't migrate down from Colorado and Utah in the 14th century with the Aztecs, but stayed behind and became the Utes, the Comanches, and (I think) the Flatheads. The language group is called Uto-Aztecan.
This migration episode is the basis of the Mexican ethnocentrists' claim on "Aztlan" as ancestral Aztec territory.
The Aztecs' migration down to central Mexico was followed, two or three generations later, by that of the Na-Dene speakers (Navajos, Apaches) down into the area vacated by the Anasazi and proto-Aztecans.