Xanathar's Guide To Something Specific So That You Can Decide Ahead Of Time Whether Or Not You Want To Include It In Your Game
Because if it's really a guide to "Everything" it had better include samurais, ninjas, psionics, gunslingers, super-powers, starships, cybernetics, mechs, glitterboys, etc.
Otherwise, it's just,
Xanathar's Guide To More Of The Same