This is a perennial subject on boards like this. I’m a Lewis collector, have been for 45 years, like unto the subject of my man Chesterton.
There are a number of approaches to this point, and you can find them in a number of places. Tolkien, as reflected in Pearce’s C. S. LEWIS AND THE CATHOLIC CHURCH (the best book on this subject from a RC standpont) emphasizes his Ulster background. Which probably had an influence early on in his development, post the return to Christianity. Tolkien’s gradual estrangement from Lewis had a number of reasons behind it, the marriage to Joy Davidman being only one (others could be speculated, beginning with Lewis’ relationship with Charles Williams, or his success as a Christian apologist, in a mode that Tolkien could not approve of, outside the RCC. And while Tolkien played a large part in easing Lewis’ intellectual problems with Christianity, he was not alone in this).
Lewis himself has expressed his principled problems with the RCC, and it doesn’t involve the concept of divorce (indeed, his words date from 1945, years before he met Joy Davidman, and the Anglican Church at the time had an even stricter approach to divorce than did the RCC), nor with being English, or Irish, of any color. In a letter to a American, 8 May 1945 (which may be read in THE COLLECTED LETTERS OF C.S. LEWIS, vol II, ed. Hooper, pp. 645-646, he explains. I don’t have time to quote it all, but what he says there is that he cannot accept what he considers the RCC additions to the universal traditions of historic Christianity. He cites the developed dogmas on the BVM, the technical side of transubstantiation, the office of the Papacy, as developed, as examples. He also said, in another place that escapes me, that it was not so much what he would be asked to affirm, as a RC, as a convert, as that he would be required to affirm whatever might be subsequently required, in the future. It is, therefore, a point on authority that he objected to.
As I said, this is a common subject, and I’ve done this before, and better. I’m in a great hurry, right now. But one point I would insist on. Lewis took his stand on principle, not on prejudice, background, or personal convenience. To suggest otherwise is to pollute the waters of the discourse.