There are all kinds of reasons. One basic example would be if the vendor has a data sheet but does not have the right to grant anyone redistribution rights to the datasheet or may even be restricted from allowing anyone to read it without an NDA. Another might be that the vendor has a general rule that their engineers don’t talk to outside people without an NDA but has decided, as a business matter, to allow the engineers to talk to a specific developer and that they want an open source driver developed. A third might be that the vendor wants to be able to have detailed conversations about proprietary implementation details of their hardware and decide later which details are going to become public.