For one simple reason: Guinness had penertrated the British market. Which meant they had a massively larger potential customer base than if their operations had been limited to Ireland.
|Dublin Porter Shipments to Great Britain 1905 - 1908|
|Guinness & Co.||604,818||650,981||670,503||687,486|
|Watkins, Jameson & Co.||38,544||39,482||36,542||36,176|
|D'Arcy & Son||23,493||27,789||23,472||21,947|
|The Brewers' Journal, vol. 45, 1909, page 8.|
Shipments to Britain from the other three other Dublin breweries were declining while those of Guinness were increasing. Eventually the trade of the other breweries would dwindle to nothing.
As you can see in the more detailed table below, in 1908 about a third of Gunness sales were in Great Britaion, the other two thirds in Ireland. When WW I erupted, the proportion shipped to Britain had increased to 40%. It increased even further after WW I, exceeding 50% in 1920.
Though even in 1904 Guinness was selling more Extra Stout in Britain than in Ireland, where the majority of their sales was in the form of Porter. Guinness actively discouraged the shipment of its Porter to Britain because they were afraid of it being passed off as Extra Stout. At this point Extra Stout had an OG of 1075º and Porter 1060º.
|Guinness sales 1904 - 1914|
|"A Bottle of Guinness please" by David Hughes, pages 276-279|
Numbers, eh? What could be more fun? Yes, obviously certain things people do in private without clothes. but otherwise, what can beat the existential thrill of scraping back the dirt to reveal a fresh number hoard? And can I come up with a paragraph containing more question marks?