* Design synergy: The historical "Triangular Trade" add value to the colonial trade by control and explotiation of the different conditions and market necessities in each continent. It has a very pragmatic mercantile reason that traduced in another massive source of riches for the European Colonial Empires, being Portugal the first, the last and by far the biggest example of this model. Others of the most notorious elements from Portuguese history like the Naval Discoveries and the Feitorias to monopolize the trade of oversea foreign tradegoods pretty much are both in real history and in-game design steps that would lead to a reason to also found your own oversea cities to produce exotic resources to export.
1- Unique explorer ship > 2- Built trade routes to foreign ports > 3- Use its benefits to expand and improve your own products
The whole process mean "complete a cycle" of naval control and trade enrichment.
* Comparative role: Slave labor in colonial plantations was not unique to Portugal but it was the main figure of this model, like Spain is to forced conversion when everyone had some examples of missionary work, or England to piracy when all others have some degree of privateering. The only other top element of Portugal is start the Age of Discovery with the most notorious naval explorers, but even in the model of Factories France, England and Netherland dethroned Portugal to achieve a control over Africa and Asia that the Portuguese never accomplished. Similarly in game just explore the map is pointless if you dont have something of value to use that territory, seize the available coast for your own cities and exotic resources would make the naval exploration more significative. Ironically unruly slave raiders like Bandeirantes (in America) and Prazeros (in Africa) were the ones going deep inland.
Like said, Netherland have a huge thematic overlap with Portugal, but also Venice/Genoa and Hansetic Leage, even others like England, Spain and Norway/Dermark just from Europe can justify strong naval and exploration elements. Lets not start with Phoenician, Arab, Javanese/Malay, Tamil, Polynesian, etc. So just naval exploration and trade are not enough to make Portugal notorious.
But what about other elements? From medieval time the Reconquista elements about resistance and religion are some of the most generic to come with since most cultures in history can justify a moment of resistance and fervor. Portuguese art and science is overshadowed by most of the main European nations, and even if one of these elements are selected there are not simultaneous and complementary to naval exploration and trade like the labor in their plantations.
* Similary dark elements: Again, Barbary Corsairs, Jenissaries, Mamluks, Conquistadors, etc. Also have pretty dark stories behind them. Still if history must be sanitised others names could be used instead of "Triangular Trade", like "Treaty of Tordesillas" since its meant the possibility for Portugal to develop this complementary colonial expansion and trade. The historic relevance of the Triangular Trade is there and Portugal is the most notorious example of it.
This is not a footnote in Portugal history, neither an isolated anachronistic element forced for the sake of represent different periods. It was a synergic element with the others of the Portuguese golden era, a world's history relevant and excelsed by Portugal. Now after all this, the design focus is not slavery, like Ottomans are not the "slavery civ" just because some slave related elements, and neither is supposed to be the ideal or "the must" design for Portugal, is only a suggestion with explained justification for an relevant aspect of Portuguese history that is poorly explored in-game.