It’s been a while since I read it, but I think there’s a couple of reasons it stayed popular (that other commenters have already mentioned). It’s a huge novel when it comes to the Modernist canon. Multiple narrative perspectives that draw heavily on the stream of consciousness technique (the first narrator/perspective of that story was, in my opinion, a wild choice that only pays off later in the novel when you “fill in the blanks”). The “fall of the south” is huge in Faulkner’s novels and is interesting to study in the context of the post-reconstruction. I’m assuming that the popularity was fueled by controversy as well since Faulkner takes on themes like race, incest, SA, and gender roles in his novels.

That said, I loved the Sound and the Fury and As I Lay Dying, but found his other novels to be less enjoyable, at least with respect to the innovative style of the aforementioned novels.