To give an example Yum! brands (KFC, Pizza Hut, Taco Bell, etc) was formed as a subsidiary of PepsiCo. Although PepsiCo has divested from Yum, their pre-existing relationship is why these restaurants only serve Pepsi's soft drinks.
Deal-making is also why you see patterns like this emerge in other places such as convenience stores that only sell beverages from the Coca-cola company (i.e. higher volume sales from just one supplier yields a better discount than splitting sales across multiple suppliers). It's relatively rarer to see more than one beverage supplier at a restaurant, club or convenience outlet.