Off hand I would guess it is because of the long-lasting radiation effects nuclear weapons. This in effect makes them the ultimate "defensive" weapons. Should an enemy invasion of your country be successful there is the option of Nuclear Suicide. In effect denying the "victor" access to your land/people/technology/secrets/resources...etc.
It seems there are some folks who would consider an invasion of Iran a desirable event in the future. Therefore it is in the current regimes best interests to develop defensive nuclear capabilities. Unfortunately some of the "Hawks" have been attempting to spin this as a reason to go to war.
The scary thing is that its working. Anti-Iran rhetoric is quite common to hear in every day American life.