as the EU explains it, the Emperor and his cronies could sense powerful jedi, and thus would hunt them down. Yoda hid on dagobah near a large concetration of darkside energy- the cave. I think Obi-wan managed to stay alive simply because Tattooine was a little off the beaten path- actually more like backwater.

then again, if obi-wan was found and killed, would have made for a very short film, wouldn't it?

as for obi-wan gathering up other force-sensitive people and training them? i don't think he would have had the recources to pull that off. at that point in the time line, the empire was pretty vast and powerful. i'm sure that if he just started flying around asking for jedi, it would eventually get back to vader. not only that, but the emperor and vader would probably notice a large concetration of force-users in one spot and send a star destroyer or twenty to wipe them out.

question? anybody ever wonder why everyone seems to have forgotten about the force? grand moff tarkin called it an ancient religion, but how long is in between AOTC and ANH? Couldn't have been that long.