The Italian connection makes sense, given Italy's domestic wine industry and its proximity to other centers of wine production across the Mediterranean.
How did Jews become so involved in wine selling? Were most of these Jewish merchants Italian as well?
Off the top of my head this does not right true, but admittedly I am more familiar with the wine trade after the 11th century. Could you point me towards the paper/book that this claim was made in?
What I can say though is that a lot of Jewish communities made great efforts to make their own wine because it was important for them for it to be kosher.
I would like to know who Rosenwein is, as well. Barabara Rosenwein in her textbook?