What does Judges 1:21 mean?

Having reported on the conquests of Judah and Simeon, Judges now begins to report on the progress of other tribes in securing their allotted territories in the Promised Land. Benjamin's territory was relatively small, but it included prime real estate right in the heart of Israel. Cities within Benjamin's borders included Jericho, Ramah, Gibeah, and Jerusalem.

The Jebusite people had taken possession of the ancient city of Jerusalem. This may have happened after Judah's army defeated the former occupants of Jerusalem and burned at least part of the city (Judges 1:8). If that is the case, either more Jebusites arrived, or they replaced the Canaanites who lived in Jerusalem before them. Judah failed to hold the city, perhaps because it was outside of their territory to start with.

When it came time for the people of Benjamin to take Jerusalem, they failed to drive the Jebusites out. Instead of persisting, the people of Benjamin simply lived alongside the Jebusites in their territory. Apparently, the Jebusites didn't have the ability to repel the people of Benjamin, either. So, they co-existed for a time. This is exactly the opposite of what God had told His people to do.

God was specific: in this situation, the depraved Canaanite culture was to be completely destroyed (Deuteronomy 20:16–17). One reason for that was judgment on Canaan's evil (Deuteronomy 7:1–5; 9:4–5). Another was to limit Israel's temptation to imitate the practices of these wicked nations (Deuteronomy 20:18). Failing to rid the territory of these evil influences led to disastrous consequences for Israel for generations to come.

Not until King David, many years later, will Israel finally evict the Jebusites from Jerusalem, which becomes the City of David (2 Samuel 5:6–9).