Together with Jericho and Ai, Hazor was the only other city that the book of Joshua says was burned (Joshua 6:24; Joshua 8:19, Joshua 8:28). We know from Deuteronomy 6:10–11 that the other cities were left intact so that they could be inhabited by the Israelites.1 Presumably, Hazor was burned because the king from that city was the one who led the northern armies in their attempt to destroy God's people and therefore his city was destroyed in a more comprehensive manner.
11 And they struck with the sword all who were in it, devoting them to destruction; there was none left that breathed. And he burned Hazor with fire.