I think if we get rich text formatting: 1) There will be an identity crisis of "is this microblogging or blogpost.md"
2) It's throwing too many (possibly confusing) features to new users
3) The timeline will look untidy like ye olde Myspace/Vampire Freaks which is never a good look
4) we've shown (and other platforms have shown) that it's not needed. Ever since Morse Code we've demonstrated plain text is enough to blast a quick thought onto a wire