Why don't most 1.5-liter wine bottles have a vintage date?

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Dear Dr. Vinny,
My local stores no longer carry vintage-dated 1.5-liter bottles of wine. The only vintage wine they carry is in standard 750ml bottles. Has there been a legal change in the way larger-format bottles can be labeled?
—Bob, Wasilla, Alaska
Dear Bob,
There haven’t been any recent changes to the requirements for vintage-designated wine. (In the United States, in order for a wine to qualify for a vintage designation, at least 85 percent of the wine must have come from that vintage; for AVA-designated wines, that threshold increases to 95 percent.) But there’s plenty of great non-vintage (NV) wine—most sparkling wine, including Champagne, doesn’t carry a vintage designation. Winemakers might choose to blend wine across vintages for many reasons, but the most common is to ensure a consistent product from year to year that reflects the winery’s house style and is less subject to vintage variation.
Most wine bottled in 1.5-liter bottles (aka magnums) comes in the form of mass-produced value wine, and it makes sense that those wines would not carry a vintage designation. It takes a lot of skill to make a large volume of good, consistent wine, and blending across vintages adds a level of flexibility in that effort to deliver a reliable product from year to year. Not designating a vintage is also another way for those value-oriented wines to pass additional savings onto their customers, as the winery doesn’t have to print new labels for each vintage.
—Dr. Vinny