Why is "3. Into China" for the Genghis Khan story so time consuming?

I've been going through and playing all the aoe2 campaigns, and I've started doing Genghis Khan's one, and it's been going smoothly, but when I got to the 3rd mission Into China, I was on the edge of ripping my hair out constantly. In no way is the mission hard to complete (I'm also playing on standard because I'd rather experience the campaigns than have a challenge). But this mission took me almost 3 hours to complete, and the only reason was because the AI would NEVER give up until I destroyed every single last building and unit. It was insane. In the other campaigns I've done so far, destroying an AI's Town Centers, Castles, and other production buildings and the majority of their army, was good enough for them to resign so I could move onto the rest of the campaign, but they never gave up until they were forced out by having literally nothing to keep them in the game. Why is Into China the only mission (that I've experienced so far) that has this problem?