Complex sentence: Those who elect to call it "Richard II, Part One" or similar do so because the play describes events immediately prior to Shakespeare's "Richard II" and provides explanations for the behavior of many of Shakespeare's characters.
Split into simpler sentences: Those who elect to call the play "Richard II, Part One" or some close varient typically have done so because the action bears a close correlation with Shakespeare's "Richard II". The drama describes events immediately prior to that history play, and also provides explanations for the behavior of many of Shakespeare's characters.