A lot of great works are not always written well. I've seen the same said about Dune. A great story that is well regarded for the story part but often cited as an example of how not to write by people teaching others to write. As people become more experienced in what is and is not considered good for writing they notice the flaws. Or if that "new car" effect wears off they might notice them as well.
Terry Pratchet references that a little bit in a quote about Tolkien.
"I read Tolkien now and notice the gaps, the evasions, all the 'bad' things... but few books have had the effect on me that TLOTR had when I was thirteen. Is he better or worse, for example, than Anita Brookner, widely regarded as a 'fine writer' although terribly dull to read? What is a writer supposed to achieve? "