More rampant speculation:
Because, they know that if they stike first they can be assured of a nuclear response which absolutely takes Iran off of the Map.
By waiting for Israel or the U.S. to strike they are arguably justified in their attack. This alone would not be enough to ward off a nuclear or overwhelming response. However, couple that with the fact that the missle will be launched by a proxy in an adjacent territory which adjacent territory as well as all people and military equipment will no longer exist after launching of the strike, and you have a frozen world body politic.
1. The nuke wasn't launched from Iranian territory;
2. All evidence of the nuke launch site, which evidence might tye in Iran (i.e. witness, those invovled, equipment and the like) no longer exists;
3. Iran was just attacked without provocation by Israel/Us;
4. It is argued or possilbe that a group sympathetic to Iran, which group acquired its nuclear weapon from the black market, launched of its own accord and not at the direction of Iran.
Given the above, Iran may indeed avoid reprisal on a wmd scale. This is their gambit. And even if they don't they still accomplish what they wanted to and to hell with the price (recall Iran has said wiping Israel off the map is worth the death of a significant portion of the muslim population. In any event, for reasons which follow logically from the state of the world they will still find themselves crippled and their nuclear ambitions dismantled in a violent way and thwarted for the rest of the forseable future.
One other log to throw on the fire - Syria moving the mines lining the border with the Golan Heights.