Just because water is safe at the source doesn't mean it stays that way if kept for long. Many, many cultures, notably China, avoid drinking fresh water because they've learned the hard way that it's not safe enough, although the Chinese solution was to boil it and turn it into tea.

Also, wine is shelf-stable and can be stored for long periods, while fruit juice cannot be. Modern fruit syrups and the like rely on large quantities of added sugar, which was not generally available back then.