The diet of most people in the US is pretty horrific. Absurd amounts of sugar, little to no vegetables, little fiber, lots of heavily processed foods.

That certainly does not help the situation. Whether it's correlational or causal I'd leave up to people more knowledgeable in the subject.

Right but as I understand the problem of colon cancer its impacting people across the board in good health - not strictly this "core" US consistuency of high fat, high sugar, low fiber, high processed food.

It is also across normal BMI, "healthy" diet and regular exercising population. Thats what's concerning about the uptick.