Why do people hate native civs?

So recently I've been learning Sioux and I really enjoy playing with them, but I have noticed a trend. Often times when I join a game the host tells me "no warchiefs". If I dare to ask why, I am immediately kicked from the lobby. It even happened when I joined a lobby called "all welcome" against a higher ranked player. I have heard that some people don't like Iroquois in 1v1s, but why Sioux? And why in other game types as well?