How Guinness became Britain's favourite pint - Stephen O'Kelly, Global Brand Director

Highest selling beer in the On Trade.
It replaced Carling, as Molson Coors shifted their focus to other, more premium brands.
Also, the decline of cask ale availability on the bars, caused many cask ale drinkers to choose their second choice - which is usually stout over lager.
Diageo - this guys employer (not Guinness - built their brand, grew distribution, than applied significant and largely unjustified price increases. They reinvest some of this price in brand support. Less commercially savvy pub operators (lots of them!) forget the high price they pay and just see the marketing support.