Why did Cardinal Ratzinger stop supporting a literal interpretation of creation in Genesis?

This is because Ratzinger professes that the Bible was constructed from pagan sources and therefore he doesn't think its true or take it literally,

Ratzinger, A New Song for the Lord, 1995, p. 86:

The pagan creation accounts on which the biblical story is in part based...”

If the biblical creation account in the book of Genesis is based in part on pagan creation accounts, then it is neither original nor inspired directly by God. It's heresy and another reason he is not a Catholic, and was not a Pope. He was the head of the counterfeit Catholic Church set up in the last days prophesied by Our Lady of LaSalette, Pope Leo XIII, Our Lady of Fatima, and The Book of Revelation.

More recently, Pope Francis has made a statement that more than confirms Cardinal Ratzinger's position. Francis said it was easy to misinterpret the creation story as recounted in the book of Genesis, according to which God created heaven and Earth in six days and rested on the seventh, saying, “When we read the creation story in Genesis we run the risk of imagining that God was a magician, with a magic wand which is able to do everything.” Pope Francis said there is no incompatibility between the Big Bang or evolution, and God as divine creator.

Notice that this position, like that of Cardinal Ratzinger, is incompatible with traditional readings of the creation account in Genesis. Undoubtedly the evolution of the Church's position has come about because anything else would become increasingly unsustainable as science moves forward in understanding the origins of the world. Even in the third century, Origen recognised that traditional readings of the creation account in Genesis are contrary to reason (De Principiis, Book 4.1.16):

.as even these do not contain throughout a pure history of events, which are interwoven indeed according to the letter, but which did not actually occur. Nor even do the law and the commandments wholly convey what is agreeable to reason. For who that has understanding will suppose that the first, and second, and third day, and the evening and the morning, existed without a sun, and moon, and stars? and that the first day was, as it were, also without a sky? And who is so foolish as to suppose that God, after the manner of a husbandman, planted a paradise in Eden, towards the east, and placed in it a tree of life, visible and palpable, so that one tasting of the fruit by the bodily teeth obtained life? and again, that one was a partaker of good and evil by masticating what was taken from the tree? And if God is said to walk in the paradise in the evening, and Adam to hide himself under a tree, I do not suppose that any one doubts that these things figuratively indicate certain mysteries, the history having taken place in appearance, and not literally...

My mother-in-law has that book you link to (she's a very traditional minded Catholic and would be aghast if the Pope invalidated the Genesis account). I read a little of it and it's more or less a collection of meditations written by then Cardinal Joseph Ratzinger, it has no official doctrinal significance and is little more than his opinion. Frankly, I'm aghast that someone could infer from a collection of meditations, written by a man who didn't even really want to be pope, something about Catholic doctrine.

Rex did a good job of giving the Catechism's take on Faith, Reason and Creation. But I've got to put a plug in for the YouCat, it just answers the questions outright

41.) Does science make the creator superfluous

No.

42.) Can someone accept the theory of evolution and still believe in the Creator?

Yes.

43.) Is the world a product of chance?

No.

44.) Who created the world?

God alone.

45.) Do natural laws and natural systems come from God also?

Yes.

There's a lot more to those answers than just yes's or no's, but it's nice that initially they don't mince the words. I'll leave it to you to get your hands on a YouCat in case you want to know more, just find someone who went to World Youth Day in Madrid in 2011 and they ought to have one in your language.

However, and to your exact point, question #46 has a more nuanced answer. I won't quote it completely since it's copyrighted, but I'll try to give you the gist of it all.

46.) Why does the Book of Genesis depict creation of the world as "the work of six days"?

The symbol of the work week, which is crowned by a day of rest, is an expression of how good, beautiful, and wisely ordered creation is.

It goes on to give 7 points, which I'll phrase in my own words.

The 7 days account are symbolic of these principles:

1. God made everything out of nothing, nothing in creation was not made by God
2. God made everything good.
3. Even things that are bad now still have some good in them ¹
4. All of creation is connected somehow.
5. Everything that was created is a reflection of God's glory.
6. There is a hierarchy in creation and more complex beings (us) are on top of it.
7. All of this points to a glorious and final end where we all be one in God.

Science is not going to inform our theology any more than theology is going to inform science. They're completely separate entities and they tell us completely different thing. Theology is infinitely more important than science since it concerns our ultimate destiny and knowledge of science is not necessary for entrance into Heaven. Science is not the realization of what is hoped for nor the evidence of things not seen. Theology is for the simple (Virgin Mary, St. Bernadette, St. Juan Diego, the 3 Children from Fatima) and it was not incumbent on them to understand complex theological principles any more than it was incumbent on them to understand complex scientific principles to receive God's grace.

If you show me one scientist who fully understands the Big Bang, Evolution and Quantum Mechanics, I'll show you one scientist who fully appreciates God's grandeur in the infinite and infinitesimal.

And that [the unheard-of precision of the processes associated with the "Big Bang"] is supposed to have happened by chance?! What an absurd idea!

Walter Thirring, Austrian physicist

Finally, I'll leave you with the words of Pope Benedict XVI (in the margin of the YouCat)

We are not some casual and meaningless product of evolution. Each of us is the result of a thought of God. Each of us is willed, each of us is loved, each of us is necessary.

Pope Benedict XVI, April 24, 2005 ²

^{1. tell your sister you were right}

^{2. Mass for the Inauguration of the Pontificate, thanks St. @ath}