Spoiler : I have had this problem all game long before AI's would have had a chance to research Banking and perhaps Ragnar closing borders with me also blocked me from Sitting Bull. But yea I think Mercantilism is a big reason why. A few AI's are still running it and Bull and Saladin just switched to open their trade and I did get a few trade routes. Do trade routes take time to build to max level? Because these are still +3 or so. I know very little about this concept of the game.