Has Zola always been adapted so much?

It seems that everywhere we look, there's a new film, play, opera or television series based on the work of Emile Zola. I'm delighted to be learning more about this masterful author, even in translation. (My French isn't there yet -- but I'm working on it.) But is the current proliferation of adaptations and reimaginings just a continued trend, is the public domain status working in our favor, or is there another factor here that I'm unaware of? Would love to hear from someone who is knowledgeable about the history of his work's popularity. Thanks!