sendos
Immortal
I managed to get this as Shoshone in my 2nd BNW game on King difficulty. You need strong diplomatic links and then pass world ideology: freedom.
I also feel it is because you are milking the current game. It seems apparent from the screenshot that the game is already over so the AI probably has no incentive anymore to switch. No one can win anymore. Try doing a new game and getting it before the game is long over and use all different civs, not all the same opponent.