‘ 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 to this day.'
The Jebusites were safe in their mountain fortress and Judah could not displace them. Yet we note that Judah at one stage did capture Jerusalem (Judges 1:8). This was probably because, with Benjamin, they captured one of its hills and its lower city but could not capture the hill of the citadel. Alternately it may be that they did at an early stage capture the citadel but had to move on, leaving it to be re-established by the Jebusites who had escaped (or been away on an expedition - compare David at Ziklag in 1 Samuel 30:1) and then returned. Later when Benjamin could have captured the citadel (note ‘did not' not ‘could not' - Judges 1:21) they allowed the Jebusites to remain rather than driving them out. Jerusalem was a mirror of Canaan, -- ‘could not', then ‘partly did', then ‘could have', then ‘failed to fully obey God' and finally ‘allowed the inhabitants to remain'.
Thus the triumphant passage ends with a note of caution. Not all was success, for Israel were not fully obedient to YHWH. As they settled down and relaxed so did their obedience. The maintaining of a high level of trust and obedience requires great vigilance and much prayer.
It is important to note that there is no suggestion that Joshua himself captured Jerusalem. He defeated her confederacy and slew her king (Joshua 10:1-27), but he did not take the city. That he left for others who finally failed in obedience.