First, this is the lowest common denominator for email.
All email clients can read plain text email.
Some older clients cannot read HTML-based email or force users to jump through special hoops to read a message. Email lists are intended to reach broad audiences. To send an email in a format in which many subscribers cannot receive alienates them at worst and inconveniences them at best. For this reason only plain text emails are accepted to many email lists, especially VSU email lists . Mailing list processing programs often do not understand requests in HTML-based email.