These tribes were on the coast of the Mediterranean Sea. They seemingly had strong trade relationship with the Philistines, who also lived on the Mediterranean coast, further south. These would not have wanted to hurt their friendly relationship with these Gentiles by joining in the war against them. This illustrates how much interrelating was going on between Israel and the pagans in those days.
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17 Gilead stayed beyond the Jordan; and Dan, why did he stay with the ships? Asher sat still at the coast of the sea, staying by his landings.