Simple reason ... Americans are way too fat! Check out the national obesity statistics here. Americans are 3 times as fat as most European countries.
Government has a role. Removing corn subsidies that ensure high-fructose corn syrup in all our foods would be a good start. (Oh, and it would also reduce starvation in third world countries which struggle to compete on an un-flat agricultural market) Town planners incentivizing more walkable communities will help. More awesome farmers' markets, like the one in my city, will help.
I personally don't even mind a tax on unhealthy foods, although I don't think its implementable.