Root beer is the only unbranded soda flavor. And while that statement isn't wholly true, it might as well be. For every soda flavor, there's a dominant brand by which you order it. Cola--coke or pepsi. Lemon lime--sprite or 7up. Orange--fanta. Grape--welch's. Actually, I've never known anyone to order a grape soda. It seems like specifying you'd like a nice, thin New England salsa.
There's also cream soda, another unbranded flavor. But cream soda isn't so much it's own flavor as it is a variation of root beer. And while that isn't really true either, my point is this: root beer is the only flavor of soda you can order without specifying a brand name. And that makes it that much sweeter. Because all the different root beers are a product of craft and competition, while the different colas are the result of increasing shareholder value.
When you drink a Maine Root Beer or a Virgil's, you can taste hints of wintergreen and clove. And when you drink a coke or pepsi, you're tasting their legal and marketing departments. I actually really like both coke and pepsi and drink them both frequently, but that's because I love the taste of legal and marketing.