Like a lot of businesses, whiskey producers are very much influenced by what their customers say, do and believe. "Conventional wisdom" says you must do this and that, and avoid these and those.
One example is bottle shape. It struck me, in looking at the new Gentleman Jack bottle (attached), and considering as well the bottles for Woodford Reserve, Ridgemont Reserve 1792, Knob Creek, Bulleit and others, even Maker's Mark, that thirty years ago, if you would have proposed one of those bottles it would have been instantly rejected. In fact, it was said about the Maker's Mark bottle as well as the SW 'barrel' bottles used for many SW brands that they were, "too fat." That was the conventional wisdom. "Retailers don't like that." The reason is that fat bottles (i.e., wide bottles) take up more shelf space than skinny bottles.
Of course, they couldn't be too skinny, because then they would be too tall, and retailers don't like that either. You don't want your product to be on the special "too tall" shelf.
One reason for the use of square bottles, and for their success, is that it was the only way to create a "billboard" on the shelf that was stronger than a round bottle, while staying within the retailer's comfort zone. In other words, a square bottle with the same width as a round bottle's diameter shows a lot more "face" on the shelf.
What changed? Probably nothing. Retailers still want commodity products in standard bottles, but if a product has something special about it that allows stores to sell it at a profitable price, they don't care how the bottle is shaped. For American whiskey, Knob Creek probably is the brand that knocked down that wall, through which all the others have poured.