I took over all the city states and almost all the competing AIs and as a whole, we only get 4 votes... You need 10 votes to win so it's impossible for any of us to win by the diplomatic victory. I had the vast majority of the world's population and I only got 2 votes? Maybe I had too many puppet states but I don't think thats the reason. I remember in Civ 4, even if there is just the player and one AI, if you had the necessary population, you could still win with the diplomatic victory. This is not so for Civ 5. BTW, Catherine is hot, I'd do her.