Because there are so many moving parts to it, and probably stems from theater days, where local troupes could do the same play, and let audiences determine which they preferred. And the theater experience was far more interactive with the audiences, and could see a wide variety set design, performance and costuming.However in theatre and movies we accept copies and remakes as wonderful, new, fresh and innovative. When did it become acceptable and why? What is the difference between redoing an old film or redoing an old painting? Why is one 'natural' and acceptable while the other is considered lesser?
Each performance was quite unique but, until we had film, couldn't be fully replicate each performance. So, it is far more acceptable to have several different performances. And that seems to carry over within the film world.