It's not a priority compared to WYSIWYG results between composing/reading mails.
The flip side of the coin is fixing HTML rendering in Word such that we can pass Acid tests, but considering the sort of mail people send, what sort of tasks Word is used for, and the fact that a working renderer is a mouse-click away, and you can see that it's just not cost-effective to take on this work.
I guess the third side of that coin is to take out Word and use something else as an email composer, but that would break all sorts of things I can't even begin to think about and is even harder than making Word a great HTML renderer (which I think is completely doable, but again, not ultimately worth it).
I'm also not on the Outlook team, so they likely have a different perspective than I on this topic, but that's how I see things.
I don't understand how composing results can be more important - who sends e-mail to themselves? I guess MS is willing to sacrifice general compatibility for it's business clients who use Outlook internally.
Word being added to the mix in the first place only reinforces that.
