Godfather II works cause it is sprinkled flashbacks throughout the movie that just tells the viewer how Vito Corleone came to power, but does not really tell a linear story where you are seeing the macro story tie into the story the preceding movie told. Godfather II has about 7-8 well done flashbacks done chronologically to give the viewer a backround of one character.
The PT is a series of movies that we know what happens, so how do you build drama with that? It is also made 16 years after the last movie and every director changes, good or worse, in that timespan, so you may get a different vision that what was suppose to be if the PT were filmed in 1985 or 1986.
I know the PT sounded cool on paper, Anakins turn, the duel, the rise of the Empire, the birth of the kids, but the problem is all of those plot points were answered in our head before we walked into the theater. Lucas should have done 7,8,9 if he was going to do anymore SW, simply cause he could have taken the story ANYWHERE, and there would be some drama in each movie cause you wouldn't know what to expect. Although if Lucas did one prequel as more as a prologue to the OT, that could have worked.