Because a committee at IBM said so. That said, the Bold is very, very heavy, and the Thin is very, very light. But I see nothing wrong with the Sans in Medium

Because a committee at IBM said so. That said, the Bold is very, very heavy, and the Thin is very, very light. But I see nothing wrong with the Sans in Medium (taking the place of bold, usually), the Text (it's not bad) and the Extralight (if you really need that, which you probably don't.) I also see that they banned #0 black text, presumably because they are elite. That's why all my browsers have Reader extensions now.