I am assuming that you are talking about the Iran Hostage Crisis, and the reason why he had negotiated was because of the recent failed military operation "Eagle Claw". At the time he did not want to risk another operation that if failed, or detected could lead to the hostages execution. Also at the time, the hostage takers were less of terrorists, and more of protesters (of the CIA placed Sha {leader}) who had suddenly taken the embassy.
