There are several good reasons for Joshua's action. First, he was wise enough to know that there would be some who would think that this was an ideal spot and stop to rebuild and live. Long live the status quo! After all, most cities in the middle east are not built on new ground, but from the rubble of pre-existing cities. After 40 years in the wilderness, this place had to look good. But the people of Israel still had far too much territory to conquer to put down roots yet.
Or it could be that he was concerned that they would be tempted to rebuild the fortress and try to hide out in it when things got tough. But God has never been inclined to have a fortress mentality. Fortresses are what people trust in when they don't trust quite enough in God. His people are, when they behave as courageously as he would have them behave, marching at the gates of their enemies.
This is an important lesson for our kids to learn as leaders of the church. The church was never designed to have a fortress mentality, a defense of the status quo, with arrows for those who dare to march outside, but within bowshot. Instead, the church is intended to attack the very gates of hell, gate by gate.
Grace and Peace,
Ron