Really? I deal with legal documents in PDF format at least once a week.
I think its to do with how it would stand up in court if a deal was challenged. There are mechanisms in place with faxes that confirm receipt of the fax by the correct organisation, so people can't just say that email didn't get to me. they are also more secure and the contents of the message can't be altered by third parties.
pretty sure there are a number of legal documents in different industries which you aren't allowed to send by email.