Did you know that Outlook uses Word to render it’s HTML?
If you’ve ever had to deal with designing an email newsletter and the responsive fallout of such a task, you may be answering yes. Or at least shaking your head with a mix of new-found clarity and complete, utter confusion.
Either way, you can read a bit about how this came to be here.
TL;DR: it was based on the belief that HTML involved ceding strategic control of the file format.