Aztec is one of the original civs and Maya are regular since they got in at CIV4. Since then many regions had gained one or even more slots, Oceania, East Africa, Southeast Asia, South America, East Europe, Anatolia+Caucasus, Central Asia, etc. Also many of the civs for those slots are alternating version to version to occupy those roles.
So, is obvious that both Aztec and Maya are so important that they are seen as must civs that are always part of every civ game. Then to open place for another mesoamerican civ we can not just replace Aztec or Maya we need another slot for the region. And looking from the trend from every new CIV game there are not total number, regional quota, cultural diversity, historical relevance, population or area justificaction to deny another slot for Mesoamerica.