Because wood makes a big difference
Any true-blooded connoisseur would know this. The kind of wooden cask in which the whisky is matured makes a big difference to its flavours. The wood imparts its character to the whisky, making it rich, light, fruity, caramel-y, bitter, etc.
Like most great inventions, legend has it that the tradition of maturing whisky in casks happened almost by accident. While the liquid was stored in wooden barrels, people consumed the liquor without any maturation. An oversight on the part of a Scotsman led to a few barrels maturing away in a cellar somewhere. And when he chanced upon this aged drink, he was gob-smacked! Or so the story goes.
Today, law requires whisky to be matured in oak casks. The toughness of the wood, its tight yet porous grain, make oak a preferred choice. It is also malleable when heated.
But why wood?
Wood has naturally occurring oils called vanillins, which slowly stir alive when the spirit resides within. Maturation helps include these into the whisky flavours.
Most distilleries around the world use one of these three oaks:
The traditionalists in Scotland and Ireland swear by this oak. Though initially Scottish and English oaks were used, they were no matches to Russian oak, which had more consistent wood structure. By the 1860s the UK started importing sherry from Spain. The sherry would come in casks made of Spanish oak, grown in northern Spain. However, the cost and logistics of sending these casks back to Spain became very cumbersome. They started using these casks to store whisky. And soon the whisky lovers fell in love with the flavours imparted by the Sherry casks.
The American oak grows fast and has tall straight trunks. Moreover, the quality of wood and the levels of vanillins make it just perfect as whisky cask. But the American oak is a more recent introduction. It came into usage at the end of the Second World War. The recovery of the American bourbon industry from the era of Prohibition, good availability of the bourbon casks and better pricing as compared to the sherry casks, increased the popularity of American oak with the Scots and the Irish.
Largely used by the Japanese whisky makers, this wood imparts truly distinct and unique flavours to the whisky. However, the Japanese oak has high levels of vanillins and is prone to leakage. That’s why many distilleries mature the whisky in bourbon or sherry casks and transfer it into Japanese oak casks.