"brewed with a single variety of barley, Optic, from the Cook family farm in Scotland (owned by our co-founder’s in-laws). The flavor of the Optic is balanced by British hops for bitterness and a UK yeast strain for a fresh, bready flavor. We age the beer in freshly-emptied Highland Scotch Whisky barrels from the Glen Garioch Distillery only 10 miles from the farm where the barley was grown."
It is also one of a few beers I know of that's been aged in Scotch whisk(e)y barrels. Why so few? Two reasons, as I see it.
The first is that barrel-aging is a "crime of opportunity." Scotch barrels are harder to come by than bourbon or rye barrels, at least for US brewers. There are distilling operations in Virginia (Wasmund's Whiskey, from Copper Fox Distillery) and Oregon (McCarthy's Oregon Single Malt Whiskey from Clear Creek Distillery), among others, making something like Scotch-style whiskey, but it is much easier to get a bourbon or rye barrel in the US, because those two types of whiskey are more prevalent.
The second is flavor profile. Scotch, like beer, starts with malted barley. But many Scotches are peat-malted, meaning that the malted barley is smoked over peat moss, imparting a (drum roll!) smokey flavor. Anything aged in a Scotch barrel that held peat-malted barley thus becomes smokey. There is a family of beers called "rauchbier," based in and around Bamberg, Germany, and indeed, Schalfly makes an excellent one of these, but smoke beers are not very popular, so it may be harder to market and sell these kinds of beer, or at least harder to match up with what one normally expects from beer. Bourbon, on the other hand, might add vanilla, caramel, and/or toffee flavors that complement a wider array of beer styles. Those flavors are present in many Scotches, too, but are often less noticeable.
UPDATE: Bartender extraordinaire and homebrewer of some note Erich Streckfuss points out a third reason: Bourbon barrels can only be used once in the production of bourbon, after which they are "retired." Scotch barrels can used over and over again, further limiting the supply.
Still, if the result is going to taste anything like this, more brewers need to do it. A smokey, meaty flavor is present, but so are notes of figs, golden raisins, and maybe even coffee, mixing sweet and savory. It's a sublime beer. Last night I paired it with lamb sliders and grilled vegetables marinated in Greek yogurt and onions. Cheers!