That is a very good question. I don't think coward is the right word though. I do think they had moments of weakness. Obi-One not killing Anakin, as has already been mentioned, was a moment of weakness, albeit an understandable one. But a moment of weakness non the less. Yoda's is another issue. Here is a Jedi Master who has just witness all that he has lived and died for over the course of 800 years be destroid, in his watch so to speak. I wonder what was going through his head as he fell to his knees in kashyyk feeling the loss of so many Jedi. He was not ready for Sidious, but he fought him anyway, and lost. I think his decision to go to exile was also a moment of weakness, but I don't know that he had any other choice at the time.