Caesar continues to be a fascinating historical figure today. I think this has something to do with the continued popularity of the play.
Caesar is the star of the Roman empire and so we go to see a play about him (or, kind of about him). Julius Caesar is perhaps the most well-known figure from perhaps the greatest civil/military empire in the history of the world - at least the Western world.
This is just one of many explanations of the play's continued popularity, of course.
There may be resonances between Caesar's rise to power and the tyrannies of the early and mid-twentieth century. I hesitate to make the direct comparison between Caesar and Hitler or Mussolini...but there may be some "thematic" overlap there.