Why would this topic be so common in modern literature and not in the literature you read before now?
If you are asking why the death of children seems to be an increasingly common topic in literature, I have several suggestions. One very well-regarded historian, Lawrence Stone, tried to argue that in earlier centuries (including the middle ages and the Renaissance), the death of children was not as emotionally devastating to parents as it tends to be today. Because there was no really effective form of birth control, parents tended to have many more children then than is the case today. Also, infant mortality rates were much higher then than now.
In other words, many more children tended to be born in the past, and many more children tended to die in childhood in the past. Stone suggested that many parents in the past simply could not afford to become as emotionally invested in their children than tends to be the case today. This argument has come under a great deal of attack, and it is possible to point to many pieces of evidence that parents in the past could suffer grief at the loss of a child just as keenly as most parents do today. (Ben Jonson's beautiful poem "On My First Son" is a standard example.)
However, there may be still be something relevant, in Stone's argument, to your question. Surely the people in modern China who are allowed to have only one child will suffer a loss of that child much more keenly than if they were allowed to have as many children as they wanted. It was not long ago that many families in this country were very large indeed by contemporary standards, and many parents simply had to take for granted the likelihood that a significant number of their children might never survive to become adults.
Thus, "A Small, Good Thing" may reflect the fact that children are simply rarer today (within single families) than they once were, so that their loss is more deeply devastating.
I think that this is a great question. I think you are going to have to offer more in terms of "this topic." There are many different places one can go with it. Are we talking about the death of a child, the disenchantment of the modern family, the alienation of the modern day worker, or how private anger can isolate individuals from social bonds? I think that "this topic" is something that is going to have to be explored.
Part of the reason why these topics are common in modern literature is that these conditions are realities that individuals have now come to accept as part of daily life. In times past, these conditions were not as pronounced and therefore the vocabulary to articulate such conditions were not as present. Given the zenith of American culture and American prosperity, it seems inconceivable that there could not be a full understanding of why a child is sick or even that a child would be hit by a nameless driver. At a point in American culture where so much prosperity is evident, Carver dissects the exact pain that cannot be avoided regardless of prosperity and happiness. The pain of a parent outliving a child, of someone being forgotten, of loss and of regret are experiences that cannot be overcome or put aside despite economic prosperity. It is here where Carver's writing and other modern writers explore themes that previously were not explored.
All of the posts above are excellent ones and give you much to think about. Basically, the answer you're getting from all of us is the taboo topics. The modern literature is now focusing on things that before no one would discuss outside of the home...media is following this example as well. Nothing is sacred anymore, and it's all out on the table for everyone to see. Disfunctionality, abuse, homosexuality, betrayal (of one's family, one's government--some today feel our government is selling our country out and working actively toward some one-world order which will not guarantee personal freedoms as we are accustomed to in this country), otherness (feeling of not belonging), and vulnerability. This last one is HUGE for Americans, as we perhaps do not feel as safe or strong as we once did--financially, jobs, technologically, or physically (terrorists, bombs, planes flying into buildings, etc.).
Today's modern literature seems to be a gross (positively used) extension of the poetry of the Confessionalists. While the Confessionalists dealt with "taboo" topics for their generation, nothing seems to be taboo anymore. Many authors seems to have thrown their filters to the wind.
Is this a bad thing?
I would have to say no. Our society is dealing with so many different problems. It is always good to know that we do not suffer alone. Knowing that there is someone else out there who has "been there, done that" helps. Reading about it, having an outlet to return to, is nice for those who feel like they have no where else to turn.
I think we need to be aware of the way in which literature seeks to reflect the issues of the society in which it is produced. This is why, when we look at more modern literature, we can see an increased focused in issues such as isolationism, disenchantment with traditions and institutions , and dysfunctional families. Obviously, you have not specified the "topic" that your question refers to, but it seems clear to me that modern fiction contains many different topics compared to traditional literature because of the changes that we see in society as a whole.
Before the eighteenth and nineteenth centuries, literature featured characters taken from the upper classes, even Tom Jones was part of the British upper class. Beginning around the times of Bronte and Dickens, social issues were taken as topics; social reform literature came into being that emphasized the commoner and the commoner's problems as opposed to the upper classes and their problems. The trend has moved ever more toward the more and more specifically common and, you might say, individual person and problem.