Albert Barnes Commentary 1 Kings 20:7

"Then the king of Israel called all the elders of the land, and said, Mark, I pray you, and see how this man seeketh mischief: for he sent unto me for my wives, and for my children, and for my silver, and for my gold; and I denied him not." — 1 Kings 20:7 (ASV)

The political institution of a Council of elders (Exodus 3:16, for example), which had belonged to the undivided nation since the sojourn in Egypt, was therefore continued among the ten tribes after their separation and still held an important place in the system of Government. The Council was not merely called together when the king needed it but held its regular meetings at the seat of government; for this reason, “all the elders of the land” were now present in Samaria. On the “elders of towns,” see 1 Kings 21:8–14.

Apparently, the king had not thought it necessary to summon the Council when the first terms were announced to him, since they affected only him. The new demands affected the people at large, and it became necessary, or at any rate fitting, that “the elders” should be consulted.