Why assume that novelty is a criterion for the worth of someone's literary output? Things can tread familiar ground, use familiar forms, and still be remarkable or worthwhile. (Imagine saying to Shakespeare: "Why write a book of love sonnets? Didn't Petrarch already do that?")

That said, I haven't read more than a few pieces by Carson and I'm curious to read more.

I try to understand what's the criterion, and inferred novelty since the other Nobel prizes seem to be awarded for some novel contributions. As I said, just trying to understand significance.

As a broad generality, you're right about novelty being a poor criterion.

But in the specific context of the Nobel Prize in Literature... The Prize's official mandate specifies an author who has "in the field of literature, produced the most outstanding work in an idealistic direction". There's also a "for the greatest benefit to humankind" criteria.

With those ground rules, writing Yet More Books in well-trod genres - but just slightly better than any of the other authors there - really doesn't seem to cut it for the Nobel Prize.