I've literally been in the room when legal and compliance offices gave the advice on both the construction of the relevant regulations and industry practices on which a payer relied on in deciding to use a process that created paper documents then faxed them for certain purposes, but, no, there's nothing published I can link to as to that being the reason industry players make that decision.

I can, however, point you to the relevant section of HIPAA regulations on which it rests, the definition of “electronic media” at 45 CFR § 160.103, specifically this bit: “Certain transmissions, including of paper, via facsimile, and of voice, via telephone, are not considered to be transmissions via electronic media if the information being exchanged did not exist in electronic form immediately before the transmission.”