They just never got anyone brave enough to simply pull out a pistol and shoot him dead.
I think most of the planners were also considering the aftermath of a Hitler death. That needed to be considered as well,m orterwise the same stuff may have continued.
1. Before being allowed into Hitler's presence, all visitors were disarmed. (Don't know when exactly this practice was instituted, but it was before the assassination attempt at the Wolfschanze, East Prussia, on July 20, 1944.
2. If the assassination attempt had been successful, Hitler would have been succeeded by Göring and/or Himmler, and the war would simply have been continued - under more-competent leadership (or the Nazi leadership would have sued for peace, claiming that Hitler was the "real" fiend, and that they had tried to moderate him).
3. By this point in time, Hitler was actually hindering the German cause, interfering with the proper execution of the war. He was surrounded by sycophants and was convinced of his own infallibility. He was aware of the fact that any truce offering from Germany would not be believed / honored as long as he was alive, and was intent upon making it impossible for Germany to enter into serious peace talks as long as he was alive (see point #2, above). Thus, killing Hitler would have the (apparently) paradoxical effect of strengthening the Nazi war effort.
Regards,