Aging wine is a complex process that depends upon the proper amount of air being allowed into the bottle. A vessel that's airtight doesn't allow flavorful compounds to develop, while one that's completely open to the air causes the wine to quickly oxidize and taste bad. You've experienced this if you've left an open bottle of wine in your fridge or on your counter too long. A properly corked bottle is just airtight enough to keep wine fresh yet allow the wine to mature: The glass is airtight, but the cork allows in .01 to .1 cubic centimeter of oxygen a day. Box wines, not so much.
"The bags used in the boxed wines that I have seen are not very good oxygen barriers," says Dr. Joseph Marcy, professor and department head of Food Science and Technology at Virginia Tech; Marcy specializes in food packaging.
Because wine companies realize customers won't be buying box wines to age, they don't bother packaging the kinds of wines that would age well into boxes, adds Marcy.
That said, one of the strong points of box wine, says Katie Vandermause, the PR manager for Black Box Wines, is that it can hang out on a counter for up to four weeks, much longer than an uncorked bottle. Bag-in-box packages have a vacuum-sealed bag inside cardboard that collapses on the wine contents as the box is emptied. That means less air touching the wine, and wine that stays drinkable longer. But ultimately the bag lets in too much oxygen over time to allow wines to age properly.