Because they are a radical Islamic regime that funds terrorism worldwide, has threatened the very existence of Israel, and thus cannot be trusted to handle the awesome responsibility of nuclear weapons responsibly.
Quite simply, one of those bombs could wind up in the hands of Al-Qaeda operative.
Lord_Baltar has decided to take a powder, so I'll stand in for him.
"Why exactly is it OK for us to have, develop, manufactuer, and test Nukes, and it's not OK for al-Qaeda to do so?"