The same things I said about the PHB also apply to splat books like Xanathar's as well. Why include niche rules that most tables won't use, when you could add more subclasses instead.that is a distinct possibility, at no point did I say that everyone would benefit from the rules
we have enough of those, there will be another Xanathar soon enough too. If they put the rules in a different book, that is fine too, it is not having them that I am objecting to
Niche rules like this are the perfect opportunity for 3PP books. Rules that are only going to appeal to 5% of players are just not worth it for WotC to publish. Their goal is to appeal to as many players as possible with their books, and to leave niche options to third party publishers.
Also, why should the niche rules that you want be included in the rules and not the various other niche rules that other people want? Its not just a matter of adding a few additional pages to the book for expanded equipment rules. Why should they add equipment rules and not additional tactical combat rules, low magic rules, or any others niche rules that other people want?