Joshua 11:11

‘ And they smote all the souls who were in it with the edge of the sword, utterly destroying (devoting) them. There was none left that breathed. And he burned Hazor with fire.'

Burning with fire was something he had only done to Jericho, Ai and now Hazor. Jericho because it was the firstfruits, Ai so as to counteract their defeat there, and Hazor because it was too powerful. It would at least render it powerless for some time into the future, although he must have been aware that people would return and rebuild it. Possibly he hoped that before that happened the conquest of Canaan would be complete. Archaeology has borne witness to the destruction by fire of Hazor at this time. In its restoration the lower city was not rebuilt.