wanna bet, farsi farter?
The downside of assassinating foreign leaders is that their people will try and return the favor. It's been some time since the era of political assassinations in the US, and the policy is to not use the presidency to order the killing of their counterparts in such a direct manner.
But like I say, killing the leaders of the Iranian regime would be simpler than uprooting a hidden nuclear program, and would scramble their government, allowing the people the chance to overthrow it.