Because it wouldn't do anything good, Un is not the entire NK government, he would be replaced and NK would almost be forced to retaliate against someone.
Why don't you(the US) just send a spy in nk kill that fatty stupid a-hole and be done with it??? (like you should have done with saddam hussen...)
Sorry if im off topic but, i have to know
As for Iraq ...
1) Before the US invaded Iraq was a seqular dictatorship, there were no WMDs and Saddam kept the terrorist groups mostly out of Iraq, there were no reason to invade.
2) After the US invasion... the country is a mess and infested with terrorists, insurgents, rebels and criminals, and this is despite the fact that the US had troops in the country and have tried to make it form a democratic government.
Assassinating Saddam would only have resulted in him being replaced by another member of his party.
Replacing one oppressive dictator with another is not a solution.
In a war however it can be a perfectly valid and sane move to try to kill the enemy leaders (to disrupt their organisation), it is important that you fill the void left behind afterwards though.