Because it sounds like java? I don't know, when does LW drink that? I can't remember that far back (will reread the game books soon, I hope).
It's mostly just mentioned in the novels, if I'm not mistaken. It's served hot, steaming, in a cup much like a coffee cup. Plus, as columbob pointed out, it's just one letter away from 'java'.

I actually used it in my Lone Wolf game. The Shadaki Bucaneer had no other reason to come into port for the first session, so I made him a hired hand on a cargo ship that imported jala beans into Holmgard. It got the party together fairly well.

Just found this in The Kingdoms of Terror:

  • As you ride west along a wide avenue of weathered red stone, you take in the early morning sights of this magnificent city. Window shutters slam open as the Varettians awake to a new day and smells of breakfast and freshly ground jala waft from shop doorways.
It's in Shadow on the Sand, too. I think it was described as hot. Between hot and freshly ground, I think it's safe to say it's coffee. That was always what I thought.