Because the international community would not be behind it and we would be at too much risk of loosing what few friends we have in the mid-East.
ALL the mid-East countries want to see Iran get the bomb, because it will be an excuse for them to get one as well. It is a way for them to participate as equals in the world community, something the Arabs have been wanting since 500 AD.
Iran will not have a nuclear bomb even if we alienate the whole Middle East. Trust me on this one, most arabs hate Iran and vice versa and they will be glad to see Iran being destroyed. The hatred between Arabs and Persians is immense.