History: Which cultures or societies went from being literate to illiterate? As in a script becoming extinct or some other reason.

This is a mythological rather than factual answer, but:

The Hmong people were illiterate, but they lived at the crossroads of a bunch of literate cultures—the Chinese, the Thai, the Vietnamese, the Laotians. The Hmong noticed. And they figured that they must not always have been the downtrodden illiterates that they were: surely they too used to have writing.

The legend was that, when they were literate, they were migrating across a river, and as they did, their horses ate their books.

Any resemblance to “the dog ate my homework” is fortuitous.

This meant that the Hmong invested much messianic expectation in the restoration of literacy. Which explains the enthusiasm with which the Hmong embraced the Romanized Popular Alphabet, promulgated by missionaries such as William Smalley. It also explains the curious history of the https://en.wikipedia.org/wiki/Pa… script—and the messianic cult around its martyred originator, Shong Lue Yang, the “Mother of Writing”.  

Smalley cowrote the biography of Yang, Mother of Writing, with Yang’s chief disciple. It is touchingly respectful.