Then Nahash the Ammonite came up, and encamped against Jabesh Gilead: and all the men of Jabesh said to Nahash, Make a covenant with us, and we will serve you.
Note that the citizens of the beleaguered Jabesh-gilead knew nothing of Saul's being made king.  This was absolutely in keeping with the detached location of their city, that, in all probability, having been the principal reason that they did not respond in the war against Benjamin.  There is nothing abnormal or surprising in their failure to know that Saul was king of Israel.