Well, because there are a lot of playstyles, the "social combat" one likely being less popular, and WotC seems very much concerned with minimizing the possible adverse effects of its suggested rules. Those featured in Xanathar's are so basic that they can't lead to unintended oversights, and that consistent refusal to publish optional rules which might split the playerbase (especially with the advent of organized DnD) seems like a new WotC policy.
That isn't to say that their overall balancing is good though, some of the subclasses from Xanathar's are absolutely nutty.