In earliest times whisky was decidedly a local product, a favourite of kings, clergy and commoners in its home territory. Production was limited to quantities sufficient to satisfy that market but insufficient to support anything but a small export trade. Vague references in medieval sources allude to its use in cementing foreign trading partnerships but there is little evidence of export in quantity before the late eighteenth century. The Lord of the Isles is known to have exported small quantities to partners in the Baltic states but it would have remained as a rare and luxury item on the tables of lords and ladies rather than finding itself a drink of the people, as it was in its homeland, the highlands of Scotland.
An accident of history, namely the union with England contrived to change this. Scotland in medieval times was a favoured trading partner with France, and had access to high quality wines and brandies which satisfied a substantial part of the lowlanders thirst. The natural enmity between England and France made this an uncomfortable arrangement and the imposition of trading barriers created the need to look elsewhere. One was a switch to Port (note the number of scottish names attached to Port brands). More important for our story was the discovery of whisky by the lowland market. At the height of the early industrial revolution, lowland Scotland embraced whisky with a passion and the industry started to grow rapidly through investment and development of industrial techniques.
It was but a short step to the English market, then the colonies and the wider world. The rest is history.