There are, probably, many reasons for this. First thing has to be tradition. The secondary whisky market we observe is a collectors market, and collectors love tradition. It is the Scots “who made whisky drinkable” and throughout the last century world famous. And the traditional scotch whisky is a single malt whisky. Next thing would be image. Many “whisky connoisseurs” believed that single grain whiskies couldn’t be that complex in taste and variety as single malts and that they are just to sweet. Although recent bottlings like the one you mentioned would certainly contradict that, it takes time to change an image. That said, maybe in the future more single grains make it to auctions and we will start tracking them as well.