my guess on the matter, apart from everything that happens in the prequels, is that there was simply no way of mounting an attack on the new empire, even in its infancy. it would seem that the clone wars were so destructive and crushing that yoda and obi-wan may have been lucky to be alive at all. Even Tarkin says in ANH to vader, "you are all thats left." and they are both suprised to see that obi-wan is alive. For yoda, it is clear that he is getting very old, and with his strenghth in the force he may have seen years ago that he was not the person to defeat vader. For obi-wan, his purpose in living on tatoine seems to be for the sole purpose of protecting luke, at a distance, which he succesfully does. in this way we can see that even if obi-wan had been strong enough to attack the emperor and vader, the small possibilty of losing the fight meant that there would be literally no hope for the jedi and the rebellion as there would be no one to train luke and the jedi would be truly extinct. Instead he stays near luke and guides the events as far as he can, and it works out pretty well. i think with all the talk in ESB of seeing the future enough to know whether or not to take a course of action, yoda and obi-wan couldve seen that an attack by them just wouldnt have worked out, and that should wait for the son of skywalker. even the emperor knows what a great threat he would be from the start.