When it comes to any field, especially when your competitor has such a big head start, you cannot come to market and not offer a significant differentiator -- in features, price, convenience, etc. If people have already invested time, money, know how, etc., into another product, and that product has already established itself as a relatively reliable and convenient one, you're not getting them to switch just because you joined the market. And the more momentum a competitor already has, the harder it is for you to gain a foothold, even among people who are buying for the first time.
In tablets, MS isn't offering enough to differentiate so far -- in price, features or convenience. And just having Bill G say something like a keyboard is a differentiator is a joke, especially when there are plenty of choices for iPad keyboards for those who want them.