The Zionist-Revisionist party (Herut) and later Likud never had complete control of the Israeli Government. The Likud always had coalition partners in order to have a majority in the Knesset (parliament.) Therefore, and as Defense Minister, there were limits as to the actions that Moshe Arens could take. In addition, I am sure that Israel did not want a confrontation with the United States Government at the time.
U.S. confirms ISIS computer expert killed in air strike (Reuters)
from the article: They fell asleep at the wheel