Sorry but isn't there a plain, simple answer to this one? 

Because of Jehoahaz's sins, God gave Israel into the hands of Hazael king of Aram, and his son Ben-hadad. Though Jehoahaz "entreated the favor of the LORD, and the LORD listened to him". But Israel did not turn away from the sins of the house of Jeroboam, and left the Asherah standing in Samaria. So Jehoahaz was left with "not more than fifty horsemen and ten chariots". 2 Kings 13:7

Jehoahaz's son Joash knew how vulnerable Israel was, and surely knew that it was his father's fault. Without Elisha's help he's in deep trouble.