Although the films don't get into too much some of Legends books talked about when is it OK for a Jedi to kill and how they kill someone can be important as well. I remember in the war with the Vong Coran Horn faces a Vong warrior and he hesitates a beat before he kills him, because he wanted the warrior to know that Coran had won. This hesitation is considered to have touched the dark side. A later book addresses a similar situation in the the killing of Luminara. So I think that is why Yoda didn't win. He couldn't have one because he has been compromised. Sidious has beaten him, out maneuvered him, killed his friends/family almost in an instant. Yoda can't win because he wants to kill Sidious very badly. I think Yoda knows that he has to try to defeat Sidious, and hopes that he is either powerful enough to overpower Sidious or has more knowledge of the force and is able to out maneuver him. When Sidious proves he his up to the task Yoda knows that he can't kill Sidious without turning to the dark side, so he retreats and goes into hiding as he has seen a different path to victory in his visions in season 6 of the Clone Wars. As a dark lord that feeds off anger, I wonder if Yoda's rage at Sidious made him more powerful or powerful enough to at least be Yoda's equal. So that is why I think Yoda lost because he wanted revenge on Sidious and that either held him back or made Sidious more powerful to even the odds. Conversely I think that is why Obi-Wan beat Anakin as he did not want to kill Anakin, which was more in line with Jedi way.