The way I understood it, Scottish distillers had widely adopted the extremely productive Coffey still and were quite willing to use non-barley grains, which all

The way I understood it, Scottish distillers had widely adopted the extremely productive Coffey still and were quite willing to use non-barley grains, which allowed them to produce a LOT of quality whisky for export so they went on to dominate world consumption of whisky. This was to the detriment of Irish producers, who at the time stuck to the more traditional pot stills and barley (some point after they got a bit more flexible). To distinguish their own product from Scotch whisky the Irish chose to start spelling what they sold as "whiskey". It's actually not so cut-and-dry as you might think regarding the USA. Even some bigger brands like Makers Mark call theirs "whisky" But anecdotally as a Scot, the only people who really get upset about that extra "e" are non-whisky drinking Scottish people :D Those who drink it are, in my experience, generally pretty chilled out and understanding.