Faxes are obsolete for a variety of reasons. The time it takes to send a fax, the various results of sending a fax and so forth all contribute.
I will focus on the legal perspective of why email obsoletes fax.
When you send an email to another person with “Return receipt requested” upon sending that email, then IF the recipient reads that email, the sender will receive a receipt confirmation that the recipient has opened that email. This is valid to use in a court of law in the United States to prove that the intended recipient did get the email and all of it’s attachments, at exactly the time the email was opened. This prevents the legal argument of “I never received…”
Fax copy on the other hand, is an entirely different matter. The only thing a sender of a fax can prove, in court, or otherwise, is that they sent the fax at a given time. It is impossible to prove that the intended recipient of the fax in question actually received the fax at all. The fax may have been sent to the wrong number. The fax machine on the recipient end may have been out of paper. The fax may have been garbled in transmission. The list goes on.
This does not take into account the cost of the fax machines, special papers, toner cartridges, the additional time it takes to send or receive a fax and so forth.
If someone does not have email in this day and age, do you want to conduct important business with them?