Charles Krauthammer gives three reasons:
(1) The nature of the regime.
Did the Soviet Union in its 70 years ever deploy a suicide bomber? For Iran, as for other jihadists, suicide bombing is routine. Hence the trail of self-immolation, from the 1983 Marine barracks attack in Beirut to the Bulgaria bombing of July 2012. Iran’s clerical regime rules in the name of a fundamentalist religion for whom the hereafter offers the ultimate rewards. For Soviet communists — thoroughly, militantly atheistic — such thinking was an opiate-laced fairy tale.
For all its global aspirations, the Soviet Union was intensely nationalist. The Islamic Republic sees itself as an instrument of its own brand of Shiite millenarianism — the messianic return of the “hidden Imam.”
It’s one thing to live in a state of mutual assured destruction with Stalin or Brezhnev, leaders of a philosophically materialist, historically grounded, deeply here-and-now regime. It’s quite another to be in a situation of mutual destruction with apocalyptic clerics who believe in the imminent advent of the Mahdi, the supremacy of the afterlife and holy war as the ultimate avenue to achieving it.
The classic formulation comes from Tehran’s fellow (and rival Sunni) jihadist al-Qaeda: “You love life and we love death.” Try deterring that.
(2) The nature of the grievance.
The Soviet quarrel with America was ideological. Iran’s quarrel with Israel is existential. The Soviets never proclaimed a desire to annihilate the American people. For Iran, the very existence of a Jewish state on Muslim land is a crime, an abomination, a cancer with which no negotiation, no coexistence, no accommodation is possible.
(3) The nature of the target.
America is a nation of 300 million; Israel, 8 million. America is a continental nation; Israel, a speck on the map, at one point eight miles wide. Israel is a “one-bomb country.” Its territory is so tiny, its population so concentrated that, as Iran’s former president Akbar Hashemi Rafsanjani has famously said, “Application of an atomic bomb would not leave anything in Israel but the same thing would just produce damages in the Muslim world.” A tiny nuclear arsenal would do the job.
In U.S.-Soviet deterrence, both sides knew that a nuclear war would destroy them mutually. The mullahs have thought the unthinkable to a different conclusion. They know about the Israeli arsenal. They also know, as Rafsanjani said, that in any exchange Israel would be destroyed instantly and forever, whereas the ummah — the Muslim world of 1.8 billion people whose redemption is the ultimate purpose of the Iranian revolution — would survive damaged but almost entirely intact.