My guess (pure speculation) is that more spam is auto-generated via email libraries and similar, and that is producing plaintext mail to try to simplify what it's getting through the filters.

On the other hand, most human-generated mail is HTML-based, as it's written in a web-client or desktop/mobile "app client", that is using HTML to let people do bold and put pictures into their email.

I was as surprised as you when I figured this out!