Remember that preview functionality is granted by contract with the publishers. Which is why some books have it and some books don't.
Almost certainly, this is something that publishers requested the removal of, under threat of requiring previews to be removed entirely.
Books that are out of copyright still have full search and display enabled.
So blame publishers, not Google.
Given the argument over LLMs consuming books illegally, I think publishers could be a little concerned that an LLM that combined partial previews on every modern work on a subject might be a destroyer of the market for the average book on the subject with the license to do so having been properly granted via this feature.
The previews are still there though, they just don't rank.
I will blame overlong copyright term lengths. 70 years after authors death or 95 years after publication, allowing most recent work to enter the commons effectively after a century, or more, from now [0].
[0] https://en.wikipedia.org/wiki/Copyright_law_of_the_United_St...