From R. Natan Slifkin, Man and Beast, pp. 203-205:
The production of foie gras started thousands of years ago with the ancient Egyptians. Wild geese gorge themselves before migrating, and the Egyptians noticed that the livers of these birds were exceptionally tender and tasty. The Egyptians developed a process of force-feeding called gavage, in which they restrained the bird by the neck and pushed moistened balls of grain down its throat. They repeated this process several times a day for several weeks, until the bird’s liver was greatly enlarged.
Although this procedure was carried out by many people in ancient times, it was amongst the Jews of Europe that foie gras became especially popular. One of the reasons for this was that it was an especially healthy food:For people who subsisted on a diet of noodles, cabbage, and potatoes, fattened goose liver was a precious source of nutrients. The Jews regarded it as a health food and dutifully fed it to growing children, since they would benefit most from the additional calories.Another reason for fattening geese was that, aside from the liver, there would be plenty of fat. This was important, as Jews did not have many options for cooking fat. Beef fat is prohibited, and butter cannot be used for cooking meat. Jews in Israel had used olive oil, but this was scarce in Europe. The solution was poultry fat, called schmaltz in Yiddish, which could be obtained in large quantities by force-feeding the birds. The fat was strained and stored for all kinds of uses, including frying, basting, moistening, seasoning, and well as an addition to cholent.
Jane Ziegelman and Andrew Coe, “A Goose for all Seasons,” Moment Magazine (June 2001)
In the twentieth century, when foie gras production moved to the United States, Israel and other countries, its method of production became more industrialized. Geese were now kept in crowded and tiny pens, and the force-feeding was done with a metal tube which was attached to a pressurized pump and shoved down the bird’s throat. The pre-slaughter mortality rate for foie gras production in Europe has been discovered to be fifteen times the average rate on other duck factory farms. The precise causes of these deaths have not been documented but are likely to be due to physical injury and liver failure. Controversies rage over how much pain and harm is inflicted upon geese and ducks in foie gras production today.
Having better understood the history of foie gras, let us now explore why, in all the halachic discussions of the topic, nobody ever objected on the grounds of tzaar baalei chayim. First, let us look at whether the process in Europe was less cruel than that of today. One might argue that the current method of feeding with a pressurized pump is more cruel, but this would not appear to be the case – the food is a soft mush that is squirted down in four seconds, whereas in past times the food was harder and would often be pushed down with a stick.
But there is a highly significant difference between the foie gras of Europe in the past and the foie gras of today. In past eras, foie gras was not a luxury, but rather was a fundamental part of the diet and provided valuable nutrional and practical benefits. Today, on the other hand, there are no significant nutrional benefits from foie gras that are not already obtained from other sources, and it is a delicacy rather than a staple. But since the concept of foie gras had long been accepted, this is probably why rabbinical authorities were not alert to the new problem.
Today, some are of the opinion that causing pain to animals is permitted for any human benefit and that foie gras is therefore permissible. Yet many authorities prohibit excessive cruelty to animals in cases where there is only trivial benefit to man, and there is a widespread custom to refrain from doing so even where it is technically permissible. Thus, it would seem that the reality today of foie gras production, where it is produced as a delicacy rather than being an important part of the diet, is not consistent with the Torah principles of hwo man should treat animals.
 Welfare Aspects of the Production of Foie Gras in Ducks and Geese, European Union’s Scientiﬁc Committee on Animal Health and Animal Welfare, December 16, 1998, section 5.4.7.
 Ibid. section 8.1.
 It seems likely that this distinctioni has been unnoticed by many who have assumed that since the rabbis of Europe did not mention the problem of cruelty to animals, there is no need for us to raise it.
 Rabbi Itai Elitzur, Tzaar Baalei Chayim bePitum Avazim, Techumin vol. 24 p. 110-112.
 See Rabbi Dr. Itamar Warhaftig, response to Rabbi Elitzur in Techumin ibid. Rabbi Ovadiah Yosef in Yabia Omer vol. 9 Yoreh De’ah 3 prohibits foie gras production due to problems with both kashrus and tzaar baalei chaim.