Nice!
I found that Xanathar's was the first non-core book that I saw a lot of "only-players" buying, and I've seen that fact lead to a lot of players taking a more assertive stance toward what the game should allow than they did before.
I'm not sure why, but if I had to guess, I think it might be that the tools and downtime rules feel player facing, even though they are actually DM facing, and it creates a strong sense of ... idk...I see a lot more people saying things like, "Well, I'm playing a tinker/inventor, and that kind of falls flat for me if I can't use downtime to make cool things, and if I can't leverage my tools knowledge during play, so those rules should be part of the game." rather than the more "DM may I?" attitude that used to be the solid majority of what I saw outside my own group.
I think that, in short, making so many books into a combination of lore, crunch, DM material, and Player material, makes more players feel like they'll get something out of those books, and also makes players feel more empowered to advocate for the game they want to play, which leads to more engagement with the game, which leads to caring more about lore and the rules in general, in a loop that also leads to more purchases by players.
That's my theory, anyway.