Just as Farmer's interest in history drove him to write, it drives Riverworld fans to read.  

But Riverworld is no stale historical review or dry commentary.  Sir Richard Burton, the star of the series, was an actual person, a real product of his age, the real-life epitome of the intellectual adventurer.  From Doc Savage to Indiana Jones, every hero with a PhD has Burton to thank for his legitimacy.

Cyrano de Bergerac, the world's greatest swordsman.  Sam Clemens, blind drunk with ambition.  John Lackland, as evil as any real person could be, so embarrassing to the British monarchy that no heir has been named John since.  Hermann Goering, a celebration of man's ability to repent.  History is the story of human beings.

History drives Riverworld, but human greatness and human weakness are what the series is really about.  Unlike so much lesser sci-fi, Riverworld takes a fantastic idea and produces a palpable narrative where character, not premise, is the center.  

This only goes a short distance to describe why people have loved Riverworld for so long.  The only way to truly understand is to read the books yourself.