The sheer scope of this one would make it nearly impossible to adapt to the screen - or, at the very least, to adapt well. Following seven generations of one family, the story is rooted in the complex history of its many characters, which means you couldn't easily remove any of them and still have a story that makes sense and has an emotional impact.
One of William Faulkner's best books (and my own personal favourite of his), but one which presents a couple of key difficulties in terms of adapting it to the screen. Like One Hundred Years of Solitude, it's the story of a family, tracing their rise and their ultimate (and complex) decay. Another problem is that much of the story's power comes from the prose style of the author, which really can't be replicated on screen.
A film version of this one was in the works for a number of years, but never actually made it to the screen. It's easy to see why if you've read it, as it is more poem than prose and its delicate style likely wouldn't survive the transfer to the screen.
Simply put, it's way too weird. Just shaping this narrative into something even remotely cinematic would take a herculean effort.
No one film could possibly contain this story. I'm not even sure any one film could contain all the story's endnotes. It's simply too big and too deep to reduce down to a manageable cinematic running time and yet still retain what makes it so great and so beloved as a novel.