As for the Jebusites the inhabitants of Jerusalem, the children of Judah could not drive them out: but the Jebusites dwell with the children of Judah at Jerusalem unto this day. (Joshua 15:63 KJV)
Why not? The Lord parted the Red Sea, made the Jordan River dry up, knocked down the walls of Jericho, and made the Sun “stand still” for 24 hours. But he couldn’t help Israel defeat the Jesubites. When Ai beat Israel in a battle, it was because the Lord was mad at them. But with Jebus, we get no explanation. Oh well, the Lord works in mysterious ways. He just doesn’t follow through on his promises.
There are three things going on in these four chapters: bragging, looting, and listing.
Chapter 12 is bragging how Israel rampaged and smote all the kings of the land, and split the territory. A big list of thirty and one kings that were smote. The other three chapters are mostly lists of borders, which tribe gets which plot of land, and some smiting. There’s also a mini-story about Caleb forcing his daughter to marry her first cousin. This is after Joshua gives Caleb promised land forty-five years late. But at least he got it!
Next: Joshua 16-18.