Hi AskHistorian,
I've recently been reading some roman cookbooks, and almost every single ingredient uses Garum, a type of fish sauce blended with wine.
http://en.wikipedia.org/wiki/Garum
How or why did that fall out of favor?
While you wait for answers, you might be interested in some previous threads on the decline of Garum in European cooking:
The End of Garum
What happened to garum? - 10 comments, over 1 year old.
The commenters link to and summarize a popular (though non-scholarly) article on the topic and also discuss its modern descendants.
Why and when did the Roman/Byzantine garum sauce disappear? - 25 comments, over 2 years old.
In this thread, users talk about the decline of the Byzantine fishing fleet as a possible cause for the decline of garum.
Other
It may live on in the form of worcestshire sauce...