A long time ago I read a writer observing that if what interest you most is human ability to make choices and act, royalty makes sense for your characters, because you can get the choices most chemically pure. If a poor man does, or does not, steal a loaf of bread, the reader is left with the suspicion that the fear of the law and hunger had a great deal of influence in the choice. Whereas, if a king steals a country, it is both more gratuitous and freer from the fear of punishment, so kings make good choices as characters to make choices -- good or bad as the plot demands.
Actual royal blood is optional, of course. What is needed is the ability to make choices and have important consequences result. Epic fantasy and space opera and many other branches of SF and fantasy allow the writer to set up choices on a grand scale. Not all of them. And the branches that don't tend to be those called literary. . . .
Unsurprising, since literary fiction nowadays does not tend to lean toward people who make effectual choices. The classic literary plot being of someone who realizes at the end how empty and shallow his life is. And I still remember a review of Incredibles that thought it was a Deep and Significant film as long as it showed Bob mired in unpleasant job. As soon as he burst free it was demoted to a mere action flick in their eyes.