G.K. Chesterton (b. 1874, d. 1936), a major figure in the Catholic literary revival, who wrote the classic Catholic apologetics book Orthodoxy in 1908 along with Father Brown stories and books shortly after (1910-), but did not enter full communion with the Catholic Church until 30 July 1922 (with first Communion and Confirmation on 24 September, taking the confirmation name after St. Francis of Assisi). He was baptized as a baby in the Church of England.
Similarly, another major Catholic thinker Mortimer Adler (b. 1902, d. 2001), a non-observant American Jewish philosopher who discovered Aquinas in his 20s and "was a frequent contributor to Catholic philosophical and educational journals, as well as a frequent speaker at Catholic institutions", only converted in 1998 after becoming an Episcopalian first in 1984. From Wikipedia:
Despite not being a Catholic for most of his life, Adler can be considered a Catholic philosopher on account of his lifelong participation in the Neo-Thomist movement and his almost equally long membership of the American Catholic Philosophical Association.
Wikipedia provides Mortimer Adler's reason for not converting earlier:
According to his friend Deal Hudson, Adler "had been attracted to Catholicism for many years" and "wanted to be a Roman Catholic, but issues like abortion and the resistance of his family and friends" kept him away. Many thought he was baptized as an Episcopalian rather than a Catholic solely because of his "wonderful – and ardently Episcopal – wife" Caroline. Hudson suggests it is no coincidence that it was only after her death in 1998 that he took the final step. (see The Great Philosopher Who Became Catholic)
But why did Chesterton wait so long? Compared to Adler, he was a lot closer already, having born in the Church of England.
As @KenGraham points out:
"The operations of Grace are known unto God alone. The Holy Spirit can offer graces to a soul, but when, how and why they become acceptable to a soul is completely known unto a God and the individual soul."
This question may not have an answer, but if Chesterton disclosed his reason, this question may have an answer that is more objective than our conjecture / opinion, as in Adler's case (see above) or in C.S. Lewis's case (see his spiritual autobiography book Surprised by Joy and the EWTN article on The Conversion Story of C.S. Lewis).
Resources that can point to the answer:
A new 2011 biography G.K. Chesteron: A Biography by "the world's authority on John Henry Newman" Dr. Ian Ker, especially Chapter 11 (America and Conversion) Section 6, covering the letters, events, and recollections in the months preceding the 30 July 1922 conversion and the immediate reactions afterwards.
His own 1926 book The Catholic Church and Conversion (pdf copy here), especially about the 3 stages in "Chapter III : The Real Obstacles", quoted by Catholic apologist Dave Armstrong blog article Catholic Conversion: Classic Analyses (Chesterton, Belloc, Pelikan).
... If, says Chesterton, he had not already been a Catholic (which he had been since 1922), here are six reasons why he would have converted. They are variable: he points to the apparent aimlessness and rootlessness of the Church of England, to the collapse of the modernist theory of progress in light of contemporary political developments, to the growing social acceptability of birth control, to state interference in English religion, to the irrationality of materialist philosophy, and, in an opaque final essay, to something pertaining to Spanish politics — the Spanish Civil War? ...
A 2002 short paper G.K. Chesterton: The Theology of Philip Yancey's Favorite Writer from the Journal of the GRACE Evangelical Society surveying Chesterton's theology and in particular Chesterton's view of Protestant soteriology (and the dearth of references in his voluminous writings about saving faith) which may have contributed to his conversion to Rome.
A book Father Brown on Chesterton by Catholic priest Fr. John O'Connor published in 1937, shortly after Chesterton's passing, containing a lot of personal details of his interaction with Chesterton. Chapter 23 has quotes from Chesterton to various newspapers about his conversion.
Possible contributing reason: from Christianity Today's Christian History article on G.K. Chesterton:
In 1922 Chesterton left Canterbury for Rome. Catholicism, he asserted, was the only church that "dared to go down with me into the depths of myself." He would have converted earlier, he told the hordes of shocked Protestants, but was "much too frightened of that tremendous Reality on the altar."
A new book (2019) My Name is Lazarus: 34 Stories of Converts Whose Path to Rome Was Paved by G. K. Chesterton which includes an essay on G. K. Chesterton's own conversion "The Chief Event of My Life" put together by the book's editor:
Editor’s note: Chesterton refers to his conversion as “the chief event of my life” in the preface to The Everlasting Man. The present essay, however, is a composite that I put together drawing from his Autobiography, Orthodoxy, The Catholic Church and Conversion, The Thing, The Well and the Shallows, the essay “Why I am a Catholic,” and a few other uncollected sources. I used this same text for the basis of an “interview” with Chesterton by Marcus Grodi for a special edition of The Journey Home on the Eternal Word Television Network.
A poem Chesterton wrote on the day of his conversion into the Catholic church:
By G.K. Chesterton
After one moment when I bowed my head
And the whole world turned over and came upright,
And I came out where the old road shone white,
I walked the ways and heard what all men said,
Forests of tongues, like autumn leaves unshed,
Being not unlovable but strange and light;
Old riddles and new creeds, not in despite
But softly, as men smile about the dead.
The sages have a hundred maps to give
That trace their crawling cosmos like a tree,
They rattle reason out through many a sieve
That stores the dust and lets the gold go free:
And all these things are less than dust to me
Because my name is Lazarus and I live.
Written the day he was received into
the Catholic Church, July 30, 1922