They are more likely to accept if you get your main stack near their last city. Not sure how close yours was.
It also helps to have a score much greater than theirs, which is normal if you took a lot of their cities so they just have 1 left.
I never played on anything less than challenging and they still surrendered plenty of times to me with just 1 city left.
Sometimes it takes an extra turn or 2 to get them to agree to it, but they pretty much always cave when I do it.