Simply because in this aspect, Microsoft can't seem to get its head out of its ass and offer what its competitors can. I can't blame GI for this. They're seeking new revenue streams, and if one platform doesn't offer the ability, it doesn't make sense to stop progress and the withhold the revenue that comes with it.
Actually MS has offered subscriptions through microsoft accounts for quite some time.
What Ivan said was they don't have a subscription service they can use which is not to
say they don't have one at all. Though I wouldn't do this for the specific set of rewards
it would be interesting to know why they can't use the sub. service. If I had to venture a
guess it is either specific for MS services or MS wants too much of the pie.