Why a back story is not enough

No doubt Colm Toibin(sorry I can't do the accents) is one of our finest writers, completely able to create an evocative scene in and to creat rich and complex characters with thoughts and feelings, characters who grow across the scope of a work as they interact with one another, learn from and change one another. He also has the alertness and sense of place and detail that make for terrific historical fiction - truly establishing the look, sense, and mood of the recent and more distant past: his novel The Master a great example, and Brookln, though I wasn't a huge fan, did capture the sense of America a century ago. Current story in the New Yorker, set in 1938 in Spain of all places, shows that Toibin is using his well-honed skills to take on a new challenge, in this case historical fiction set largely during the Spanish Revolution - an area you'd think had been written of to death back in its day, but apparently we're not done with it, and Toibin brings a fresh insight: story involves an elderly woman, widowed and alone (daughters relatively nearby) in a small Spanish city; one of her daughters puts her in touch with a guy in town who's writing about the history of the village, especially its time during the Revolution, and he has met a former Franco general who will come back to the village to speak with him for his book - and the General would like to have lunch with the elderly woman. Then, we go into the back story, as we learn that she had a romance with this general, then an ordinary Franco (Loyalist?) soldier back in their youth, which led to a pregnancy; she later married a guy from the village whom she, at first, didn't love to cover up the source of the pregnancy - she came to love her husband over time, and now he's gone. She declines to see the General; end of story. Well, this back story is very well written and has a lot of potential - we've seen other fiction about French women who consorted with German occupiers, but I hadn't seen anything about Spanish women who cozied up to the fascists - I was intrigued. But unfortunately this story stops dead in its tracks: what's interesting or potentially so is not the back story but how the story can be brought to the foreground. Was the woman ashamed her whole life, or scorned in any way, or did the village not know of her affair (unlikely) or were they all in some way complicit and silent about that? The potentially great scene would be her confrontation with this man - and her taking stock of the course of her life, and that never happens. A back story is just that - in this case, it's just a premise for a story  - and I think Toibin owes us more or missed a chance for developing a great narrative. And maybe he does do that - maybe this is a piece of a longer work in progress.