But surrounding himself with "yes" men during the writing of the PT, much of which was still written on the fly throughout the filming, makes it much easier for such problems to creep in. Try reading the Annotated Screenplays sometime, and see how much back-and-forth there was between Lucas and his group. This is what kept everything in focus, especially for ANH (the early writings were quite laughable.) ROTJ, my least favorite of the OT, is when Lucas really starts going solo, leading to two death stars to begin with, and a rescue sequence that foreshadows his PT ideas about reasonable cause and effect.
"Plus, Tarkin likely assumes that Obi-Wan must have been killed in that 19 year time period between ROTS and ANH, given what we know about the empire's hunt during that time, for the remaining Jedi who survived the purge."
And the fact that he, as well as Yoda, have apparently done nothing in the intervening years. In the novelization, Ben hints at other surviving Jedi who would probably not bother to assist Luke if he needed it.