To say that we drink Pepsi products exclusively is a gross exaggeration. It's funny you should be asking that question as I am forcibly drinking a Coke right now... I forgot my normal Pepsi ration at home today and Coke is all that's available in the vending machine downstairs. After a bit of research, I do see however that Pepsi has a larger market share here VS Coke... larger per capita than anywhere else in the world.
I know I have read on various occasions that at some period in the last 75 years that Pepsi here was less expensive that Coke. This may have an influence over the culture... I would imaging this may be the reason; the notion of buying a less expensive beverage may have stuck.
Why do I drink Pepsi? It's a matter of taste preference. The two taste different and I prefer the cold, crisp taste of Pepsi. I'm sure some of you would have a laugh when I arrive to Atlanta to my yearly pilgrimage with 12 710ml bottles of my beloved Diet Pepsi.
, and thereby causes decreased "close combat" 
effectivness of ALL USCM, UACM and UKCM 
).
Coke= 
...I think Burke has been sneaking them Coke....