MAD only works when each side is convinced the other will use them if provoked, and that there are rational folks in charge who will make sure niether side will start the stupid.
The Iranian leadership are not convinced we will use the nukes if they do. And even if we could convince them that we had the will, they don't care. Let us turn the whole place into green glowing glass, they wouldn't give a rip, as long as they get in first licks against the enemy de jour.
Anyway, who would we use it on? Tehran is full of their enemies.They would survive, their enemies would not. Win win.