To illustrate radarskiy's point, one can't always use "one" in some contexts. For example, "I'm indeed acquainted online with a person named Sydney, but I honestly don't know their gender" is not a statement in which we can s/their/one's/, though one may s/their/that person's/ if one wishes to avoid epicene "their".
I've seen another comment suggest that it's because MS is afraid of repeating Win XP's scenario where many computers remained on Win XP for a long time over again.