In my experience, most mails don't actually use any styling beyond the automatic blockquote for the endless quote chains that mail clients automatically add. So people are wasting storage and transfer for something they don't really use.
When I see a mail which could actually use some styling, it's usually something involving maths and HTML is not suitable for that case. People usually send LaTeX snippets in plain text in this case.
And while I'm at it, the endless quote chains are another thing which has no reason for existing. Each mail in a thread steadily grows in size, because mail clients "conveniently" always quote everything and put the text cursor above it so that the user doesn't stop for a moment to look at the size of what they're sending and consider the absurdity of the situation, when their client is perfectly capable of reconstructing the whole thread without copying all mails in each message. Quoting should be opt-in on a case-by-case basis. As they function in the wild today, the quotes are just useless garbage attached to everything.