Amazon requires DRM, which Doctorow considers anathema. Since the author won’t allow DRM on his works, Amazon refuses to sell them.
In fact, the author, in order to be found in Audible search results, penned an essay explaining how Audible rips off writers and readers. It's called "Why None of My Audiobooks Are For Sale on Audible".
You can read via the link following:
https://pluralistic.net/2022/07/25/can-you-hear-me-now/#acx-ripoff