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.
no it is not for geeks. It is not a payment method at all anymore and this is why they removed it.
It is amazing how people can deceive themselves and keep deceiving whatever happens and whatever facts are in their faces.
Here you have the
true reason it was removed:
They removed it because making payments in Bitcoin was no longer viable. Who want to pay a transaction fee of $100 for purchasing a Microsoft Office Suite which costs $150? The charges levied by Visa/Master for the same is around $3 per purchase.