Exactly. Why is Valrhona's Manjari so expensive when it's not even a great example of Madagascan chocolate (at least not at that price level)? I currently live in Dubai and Valrhona's prices are ridiculous in that they are out of sync with their quality . Felchlin's Maracaibo 65% is superior to Manjari yet considerably cheaper. The phrase "Grand Cru" in the chocolate industry more often than not is weasel-speak for I'll-give-this-slightly-better-than-average-chocolate-bar-a-fancy-schmancy-sounding-appellation-cum-classification-and-take-the-consumer-all-the-way-to-the-cleaners-(laughing heartily as I do so)!
Best answer for this question is "brand inflation." Of all the chocolate makers, Valrhona has done the best job of positioning itself as "the" premium brand for confectionery and pastry.
And you pay for that.
Is it the best? Matter of personal taste. But you pay for it, nonetheless.