Though Palestine was made up of independent and hostile city states at this time, the presence of a common enemy caused them to rally to Jabin’s call.1
Also, the listing of the kings is meant to emphasize the dangerous and almost hopeless situation in which Israel finds itself. This is to make the power of God shine brightly.2
1 When Jabin, king of Hazor, heard of this, he sent to Jobab king of Madon, and to the king of Shimron, and to the king of Achshaph,