I’m going to take a wild stab here and say that Mexico consumes a lot of soft drinks because, (a) it’s really hot in most places and (b) the water is often not so hot. I know when I’m there, I drink way too many Cokes. I could drink more beer, but then I get confused and want to stay there forever.
As for massive consumption in the USA, my suspicions lean toward saturation advertising. It’s hot in the Southwest and South, too, but our water is generally fine. We drink cokes, generically speaking, ’cause we’re trained.
But from a health standpoint, I agree with Reid and his pal. Order water. Or at least ask for iced tea.
