Answer:
Off the top of my head,
Experience defines what you believe in: It is possible that Shakespeare benefitted from Monarchy in the form of bountiful patrons, while Fitzgerald might not have had good experiences with the upper class (which is not so hard to believe)
Loyalty and allegiance might cloud your judgment: Here, I'm considering Fitzgerald's point of view as more objective because, well, refer to pt. 1. Shakespeare wrote a great deal of drama on historical figures, and it just so happened that he wrote (or portrayed) not so nice things about Queen Elizabeth's rivals.
Time period plays a significant role: During Shakespeare's time, the idea of "divine right" was prevalent, which stated that Monarchy is chosen by God himself. In such a situation, the idea of Monarchy as righteous, just, and reverent echoed with the masses. Whereas by the time Fitzgerald started to write, Monarchy was discontinued, and protests and executions of the Upper class for their treacherous actions were conducted. All in all, Upper-class' sanctity was greatly diminished in the psyche of the masses.