The standard answer to the question "why wasn't such-and-such put directly
in the standard" is "there's no reason why it has to be in there -- this is
an extensible registry we're talking about here". If you think MicroSoft
RTF is important to your needs you should write an RFC describing it and
register a subtype for it.
Please use the mechanisms for their intended purpose rather than worrying
about what's in the base document and what is not.
As for the question of why RTF wasn't used instead of defining a separate
richtext, I'll let Nathaniel answer that part if he so chooses. My personal
take on it is that MicroSoft's RTF seems to mean different things on
different platforms, and I certainly didn't feel comfortable with the
notion of trying to profile it for use in MIME. There is nothing to stop
you from doing so and showing us all how easy it is, however.