Because of geographic spread and the playerbase's obsession with the Iroquois. As long as the Iroquois are a civ, the instinct is to forgo the Sioux and include a Southwest civ instead. And now that we have the Cree, another roaming plainsfolk, I think the Sioux are also unlikely. Although they have the territory and population to be a frontrunner, if we only got one more native tribe the perfect trifecta is clearly Cree, Mapuche, and Navajo. In terms of political organization, land ownership, population and heritage, mechanical and geographic uniqueness, the Navajo trounce literally every other American option. It's not even close. Now if we got a fourth North American tribe, then I think the Sioux stand a chance. But they would still be competing, with the Inuit, the Haida/Tlingit, and the Hawaiians. And if by some unfortunate turn we got the Shoshone instead of the Navajo, then I would consider the Sioux dead in the water because the Shoshone kind of fill both the plains and desert niche. In a perfect world where we got all the major tribes, we would have Cree, Tlingit, Navajo/Pueblo, Sioux, Anashinaabe, Iroquois, maybe Cherokee/Choctaw and/or Shoshone. But that's not happening and the design space for the Sioux is already someone infringed upon. I don't think it's likely, and as far as having a good generalist representative in a game that can only do so much I think the Cree do a decent job of vicariously representing the PNW, Great Lakes, and Plains tribes. So Sioux are pretty meh for me. I wouldn't dislike them, but under the VI paradigm which is really favoring the modern survivability and influence of cultures rather than imperial sprawl, I would favor the Navajo as a political entity and minority group, the Hawaiians as a larger native population, and the Inuit for basically controlling Greenland and having a unique playstyle, before I would start looking at the Sioux.