From Martin Wiggins' Shakespeare and the Drama of his Time (OUP, 2000):
Though Roman plays [in the Elizabethan drama] ranged widely from the last days of the Tarquins in The Rape of Lucrece (Heywood, 1607) to the dissolute later emperors in Heliogabalus (anonymous, 1594, lost) and Valentinian (Fletcher, 1614), the greatest concentration of attention was on the middle to late republic, and in particular on the Punic Wars, the conspiracy of Catiline, and the career of Julius Caesar, the single most dramatized historical figure in the English Renaissance. Unsurprisingly, these were also the subjects covered by the three Latin historians most often studied in the Elizabethan grammar schools, Livy, Sallust, and Caesar; and while the playwrights generally did not use these school texts as sources, the familiarity of the events and characters must have been a key factor in the selection of subject matter (21).
I wrote a piece for the Guardian a while back, expressing frustration that the same Shakespeare plays kept coming round, catering to perceived educational needs. While I knew what Wiggins says here, however, it's the way he phrases it which caught my attention just now. In a sense, this is the same problem that's been with us since the earliest days of modern English drama: the familiarity of stories ensures their constant re-presentation in various forms. One wonders if early modern audiences ever got fed up: "Oh no, not bloody Caesar again, I hated Latin class..."