No, it’s because there isn’t enough physical evidence to support the idea that it was a regular component of the everyday diet. And Diaz’s account is not necessarily any more accurate than that of modern day eye witnesses.
In a dig you will encounter the remains of meals in the hearths and in garbage pits. Analyzing these remains you can get a rough idea of the everyday diet of a household in a layer of soil, and by the percentages of species’ bones found get an idea of what the most common meal was, within reason. Large bones like human bones or deer would be cracked to obtain marrow, so these would be brought home where other less choice parts might not be. By the postholes or foundation stones and other items inside, you can determine if a house was a peasant’s or a nobleman’s.
The Aztecs left records and among these as I recall were illustrations of a typical marketplace with all the foods and other goods depicted, like peppers, fish, feathers, turkeys, etc. There weren’t rows of human hams in the scene...
http://www.foodtimeline.org/foodmaya.html
Good explanation.
And, as I mentioned, Diaz writes about a handful of humans at the major cities. So that would be consistent with the lack of bones in ordinary middens.
Another thing mentioned is large numbers of stacked bones, so perhaps human bones were treated differently than other bones. Maybe cannibalism was limited to the upper classes.
Also the numbers of humans sacrificed only come to a very small number per person per year.
Just a quick calculation shows even 100,000 people sacrificed and eaten a year becomes only a tiny part of the yearly caloric intake.