OMG! I did research on garum in my undergrad! As I’m sure you know, garum, liquamen etc. was the most popular condiment of Roman cuisine. It’s origins are unknown but it is believed to have originated in North Africa. It was made using undesirable parts of a fish as well as fish too small to eat. It was fermented using vast quantities of salt and left in earthenware containers to bake in the sun. The smell must have been wretched, so the places chosen to create this sauce were usually placed outside of the city and preferably down wind. Pompey was a known for its garum production and the sites in which the garum was produced have been uncovered. (Pompeii and Herculaneum: a Sourcebook , Cooley and Cooley, pages 247-251)

Anyway, to answer your question, we have to look at Roman trade. Rome, at its height, traded with India, however, other than gold, there was not much in commodities that Indians desired. Garum and olive oil were very popular trade goods that Indians sought after. Eventually Indians learned how to make it for themselves and spread it through east asia. You can try garum at nearly any Asian restaurant but you will see it under the name “fish sauce.”

Why did the Romans stop making it? Well there are only guesses but it was probably due to Roman decline. To create garum you need plenty of fish, salt, and time. As the empire started to decline, so did trade routes. Less trade, less demand for luxury items such as garum. Getting a hold of salt also became more challenging as trade started to decline. As the political stability of Rome more and more questionable the investment of creating garum became less profitable. (Basically, if you are afraid a band of German goths are going to pillage your town you couldn’t stick around to protect your fermenting fish) now there are pockets of coastal Italy that still make fish sauce so it didn’t die out completely, it has just never held the same appeal as it once did. (Fish: Food from the Waters, edited by Harlan Walker, From Garum to Ketchup, Andrew F. Smith, pages 299-304)