Plans to kill Hitler were cancelled because people were afraid his successor might be more rational and effective.

Strategically deciding to make the kill on a big player in a government, generally a government leader is a hard decision to make. One major assassination that ended badly was the killing of Reinhard Heydrich. 

He was the leader of the Third Reich as it existed in Czechoslovakia, and inspired quite a bit of terror. It was his assassination that led governments to be more careful about assassination in the future. 

Heydrich was killed by Two Czechoslovakian men who parachuted in. Records had it that they had been trained in how to best assassinate the man, and also planning to continue through with the assassination even if it meant their own deaths. 

The assassins made their attempt when Heydrich was in an open-top car, and the injuries that he received killed him. 

As a result of the assassination, Nazis responded with more terror: killing men, and putting women and children in concentration camps. Historians in general agree that his death caused more pain than it saved. 

Perhaps for this reason plans to take out Hitler were stopped. The biggest fear was that whoever replaced Hitler would be more effective and logical and better able to defeat Britain. Besides that, is it ethically acceptable to take out specific political enemies in a war? 

(Source)