The New York Times carried an obituary the other day of Josef Burg, a Yiddish writer who died in August at the age of 97. Born before World War 1 in the Habsburg empire, the world he was born into was destroyed before he went to school. The world he grew up in was destroyed by the time he was a young adult. The world he spent his middle age in has been gone almost a generation. Dramatic change, you might say, was boringly constant in his life. It fit comfortably into his main occupation, which was to write in a mostly dead language about a vanished world to a public that no longer existed. Ironically, the road to having an obituary in the NYT went through the German language; late in his life his Yiddish manuscripts were discovered by a young German, translated, published, and rather widely sold in Germany, of all places. (This is actually not as surprising as you might think. There is far more genuine interest in things Jewish in Germany now than there was when Burg was born).
Not long ago I read Aaron Lansky's fun book Outwitting History: The Amazing Adventures of a Man Who Rescued a Million Yiddish Books. Lansky, an American Jew with a tenuous connection to Yiddish until college, set out in the 1970s to find Yiddish books to read, realized they were mostly to be found in places with no future such as the homes or social clubs of elderly Jews whose children were not interested, and decided to gather them up. The experts he consulted told him there might be as many as 70,000 volumes to be found if he was systematic; he and his friends ended up collecting about 1,500,000. It's a poignant book - I read long stretches of it laughing and crying, both.
The people he met along the way are just about all gone by now, but so is their world, in a way far more profound than the mere passage of time would explain. Three main things did it in. The first, chronologically, was the Shoah, which destroyed the Eastern European heartland of the Yiddish world - for Yiddish, as a thriving culture, was an invention of 19th century Eastern Europe. Previous to that it had been a living language of the Jews for about a thousand years, but it hadn't been the language of their cultural creativity - that had been Hebrew, or Arabic, and near the end, German.
Second, the Yiddish world didn't survive the emigration to America. At one point Lansky notes that a significant majority of Yiddish books ever published, were published in New York. Yet the Eastern European Jews who came to the New World and the Goldene Medine migrated so their children might have better lives in the land of the free. They were wildly successful in this, but at the price of putting the new land above just about all aspects of the old country - and Yiddish was about as Old Country as possible. So the immigrants' children had no use for Yiddish, their children never knew it, and their children - mostly - neither know about it nor care in the slightest. The world of Yiddish wasn't strong enough to compete. (This goes for other lands of immigration, too - the English speaking ones, but also the French, Spanish and Portuguese ones).
The third reason the Yiddish world disappeared is that it's roots were too shallow to thrive in Israel. People who know better than I tell me Yiddish is a better tool for studying Gemara than any other language, with myriad turns of tongue that can explain an entire complicated passage in three or four words. (A teacher I study with often uses "shrek fun allen seiten" to clarify a knotty problem, but we're not really using Yiddish; it's merely a remnant). But faced with Hebrew as a living language for all parts of life, Yiddish never had a fighting chance. After 1948, when the largest group of Jews in Israel were from Arabic-speaking lands, it wasn't even in the competition.
Of course, there still is one vibrant corner of the Jewish world which still uses Yiddish. Part of the Haredi community, in Israel and in the US, still raises its children in Yiddish and speaks it at home and in daily life, much as it did for a thousand years. But it's only part of the Haredi world; and anyway, as Lansky show very convincingly, the Haredi community isn't part of his story. Read his book to find why.