A significant number of civilizations now considered staples in the Civilization series were not playable until Civ III: the Dutch, Koreans, Incas, Mayans, Ottomans, Byzantines... Even the Arabs, despite their immense historical significance, could not officially be played until Civ III.
Portugal is always included later for a simple reason: to avoid overlap with Spain and the Netherlands. Only when they decide that the other regions of the globe are reasonably represented, then they decide to include Portugal. This is understandable, as having Spain, Portugal, and the Netherlands included too early might have felt disproportionate while other regions still lacked representation. However, I’m unsure if this rule will apply to Civ VII. The Mayans, for example, have also traditionally been introduced quite late—always after the Aztecs and Incas. Now, we see the Mayans in the base game because they are the most fitting choice for representing antiquity Latin America. Similarly, Portugal's inclusion could be anticipated for a comparable reason: their gameplay aligns seamlessly with the Age of Exploration. After all, which country initiated the era of maritime discoveries?