This is just a WAG too, but I think it’s a pretty good one.
I’d suggest: rarity and exclusivity.
Wine has been around forever. Grapes (and other fruits) have been fermented into alcoholic beverages throughout recorded history. Wine’s actually pretty easy to make, although good wine is more difficult. Some vineyards in Europe go back hundreds and hundreds of years; generations of tradition have taught the winemakers how to grow specific grapes in specific climatic zones, and then how to turn those grapes into wine in order to get the best possible flavor from them based on everything that went into growing them: length of season, type of soil (known as “terroir”), and so on. Still, wine in general was hardly an exclusive commodity.
But then the mad monk Dom Perignon (recognize the name?) developed the “methode champagnoise,” i.e. the means of making champagne. To do this, you ferment your wine the same as you would any other, but then you transfer the wine from the cask to the bottle and re-ferment in the bottle to create the carbonation. Then you add a few other ingredients (such as sugar) to stop all fermentation, leaving a certain amount of carbonation in the wine, and cork the bottle.
Way back when, this was an extremely difficult procedure, and Dom Perignon jealously guarded his secrets. Consequently, the champagne product was very, very hard to come by. You can see where this is going: If something is rare and expensive, it will be considered very special, and will be consumed only by the wealthy on special occasions.
As the methode champagnoise spread, it became available to the wealthy on a more regular basis, but among the plebians, it would then be the same sort of special-occasion rarity.
That brings us to today. First, it’s important to realize that the democratization of winemaking is a fairly recent event. These days, you can go down to any grocery store and find a good ten-dollar bottle, comparable to some of the better wines from two hundred years ago. (St. Supery makes an outstanding Sauvignon Blanc that retails for twelve dollars.) And for six or seven bucks you can get a bottle of cheap champagne, say an off-label production of Gloria Ferrer or Freixenet. Because this happened so recently, though, champagne still has that certain cachet.
Of course, there are much nicer champagnes available. For fifty bucks, you can get a very nice White Star, or maybe a Veuve Cliquot. And a bottle of Cristal can run two hundred dollars or more.
So, based on history: Rarity, exclusivity, and expense dictated that champagne would be a “special-event” consumable, and the tradition holds fast.
Not bad for a WAG, huh? 