Possibly because the channel on which the "fax" would travel (e-mailed PDF) is not the same as an actual fax (although nowadays it is probably going to be over

Possibly because the channel on which the "fax" would travel (e-mailed PDF) is not the same as an actual fax (although nowadays it is probably going to be over T.38 transported over IP, so in the end probably not much changes in terms of security). If those PDFs were properly encrypted using a widely available PKI, then it would be a better replacement for an outdated technology. There might also be legal requirements preventing the unencrypted transmission of some type of documents (e.g. court documents) over the Internet. There is also iFax, which is supported by several vendors, which is essentially like a fax over e-mail. Many multipurpose fax/printer/scanner machines support it.