Keep in mind that in ye olden days, email was not a worldwide communication method. It was more typical for it to be an internal-only mail system, running on whatever legacy mainframe your org had, and working within whatever constraints that forced. So in the 90s when the internet began to expand, and email to external organizations became a bigger thing, you were just as concerned with compatibility with all those legacy terminal-based mail programs, which led to different choices when engineering the systems.