Because US government is against cryptocurrencies. As long as Microsoft doesn't want to have problems with officials they won't take any risks. Especially risks like that: Bitcoin as a payment method is only for geeks and it's not worth keeping it just to satisfy them.
$100 transaction fee? Did we reach the moon already?
Fees can be as high of as low as you make them (of course there is a risk of lowers fees for tx's never being accepted), or stuck in the mempool for days, but come on - $100?
Absurd.