There is no particular reason to believe they did. Just because we don’t know where letters are doesn’t mean that they have been destroyed, and if we don’t know for sure they’ve been destroyed, we can’t attribute motive to any particular person for destroying them.
Despite what you may have read in books, in magazines and newspapers, or on the internet, there is no evidence that Willa Cather made a regular practice of demanding the return of her letters so that she could destroy them. Likewise, the evidence of Lewis gathering up and destroying Cather’s letters after Cather’s death is very limited. Indeed, she was responsible for the preservation of many letters. I discuss and fully document these claims in an article forthcoming in Tulsa Studies in Women’s Literature in 2021, but I can offer some examples and details here.
There is one clearly documented instance of Cather’s destroying a significant number of her own letters, namely her letters to Isabelle McClung Hambourg. Isabelle’s husband, Jan Hambourg, sent these letters to Cather after his wife’s death in Italy. There is no evidence, however, that Cather demanded that Jan Hambourg send her these letters.
From this example, some make the leap to Willa Cather and Edith Lewis routinely destroying their correspondence with each other. Cather, however, never faced Lewis’s death, as she faced that of Isabelle McClung Hambourg. Lewis outlived Cather by twenty-five years, so she did face Cather’s death. At the present moment, only one letter and five postcards from Cather to Lewis are known. I derive the title of my book from that one beautiful letter, which you can read here.
Even though they lived together, Cather often traveled without Lewis, so clearly she wrote Lewis more letters than this one. So where are they?
I wish I knew, of course, but I’m not convinced that Cather and/or Lewis destroyed them. The fate of that surviving letter and the postcards is interesting. The letter is in the archive of the National Willa Cather Center in Red Cloud. It’s not clear how it ended up there, but I feel certain that Lewis herself did not send it there. She opposed the creation in the 1950s of what was then called the Willa Cather Pioneer Memorial because she believed that it would not have pleased Cather. So how did this letter exit the Park Avenue apartment which Cather and Lewis shared from 1932 to 1947 and where Lewis continued to live until her death in 1972?
Similarly, how did the five postcards end up in the hands of collectors, who then donated them to libraries? I have some theories, but the continuing existence of these pieces of correspondence suggest that Lewis did not make a bonfire of Cather’s letters to her after her partner’s death. Furthermore, she paraphrased parts of Cather’s 1936 letter in her memoir of Cather in Willa Cather Living, which was published in 1953. This paraphrase suggests that Lewis still had this letter in her possession at least into the early 1950s.