...Whatever the reason is I am sure Microsoft would only adopt something due to the massive savings it offers them in transaction fees, convenience, accessibility, or whatever the case is....
Since the transaction is done through BitPay and subject to their fee, whatever financial gain (savings) they may see on a per transaction basis is likely not massive at all.
Microsoft did it primarily for two reasons...
1) Positive PR - gives the public the impression MS is forward thinking in 'keeping up' with innovation
2) Never be negligent in opening a potential avenue for revenue when the cost to implement a new payment option is practically nothing