It seems like a health safety issue for allergies and for diabetics. In general, it seems like it would be better for the public if they knew the ingredients and the nutrition facts of alcohol products. What laws allow this loophole and is there any history behind this with the alcohol lobby/other companies? How do current organizations like the FDA justify it?
In the United States, alcohol is regulated by the Bureau of Alcohol, Tobacco, Firearms and Explosives within the Department of of Justice (historically, it was part of the Treasury Department reflecting a historical focus on tax collection). Alcohol is exempt from the jurisdiction of the Food and Drug Administration (FDA) that regulates most other food products.
The FDA was organized in 1927. Prohibition was in place from 1920 to 1933, so at the time the agency was created, giving it jurisdiction over alcohol would have been like giving it jurisdiction over cocaine or marijuana. Alcohol was considered contraband and wasn't in the same category as ordinary food and drugs. And, when prohibition ended, the regulation of alcohol, tainted by its prohibition history, was not seen as something in the same category of the technocratic health regulation of food and drugs. It was a job for an agency that could keep former mobsters in check, not one for chemists in the agriculture department.
There are rather elaborate regulations with all sorts of historical baggage that govern these regulations.
For example, it is mandatory for labels on some kinds of alcohol to disclose the percentage of alcohol in the product, and prohibited for other kinds of alcohol to disclose the percentage of alcohol in the product. Certain ingredients, like nitrates, have to be disclosed on the label, while others do not. And, many of the primary ingredients of alcoholic beverages with a particular name, are dictated by ATFE regulations.
For example, in order to call liquor "bourbon", at least 50% of the alcohol content must be derived from corn derived alcohol that is distilled and then aged for a certain period of time set forth in the regulations.
Similarly, to be called "wine" a product must be derived from fermented grapes, and to be called "brandy" a product must be derived from the distillation of wine. Another defined term is "malt liquor" (which is a form of beer with added alcohol). "Gin" must be derived according to regulatory specifications from juniper berries.
Also, there are elaborate regulations regarding what particular name can be given to particular types of wine (for example, what percentage of what kinds of grapes must be present to call a wine a "Merlot" or what a wine must do to carry a geographic identifier).
These elaborate alcohol specific labelling laws largely pre-empt other labelling laws and trademark laws of general applicability in the cases where they apply.
Historically, alcoholic beverages have not had a very complicated set of ingredients and those ingredients are usually strongly (although not completely) determined by the name, so ingredient labelling has historically not been a big concern.
There have been efforts to reform alcohol labelling regulations to require nutrition labelling of some sort with alcohol, but generalized warning labels of the kinds found today (e.g. for pregnant women, and for drinking and driving) have largely been a higher priority.