Are you serious? Do you really think thats the reason that this is happening -- that people don't just eat their veggies? Fiber is important but, um, that's a pretty hot take.
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.
No health issue is so easily reducible, but the impact of eating a diet of "actual food" and moving around even a little bit daily cannot be overstated.
The odds ratios for nearly all diseases and all-cause mortality shift so far from those two interventions it's almost unbelievable.
That certainly does not help the situation. Whether it's correlational or causal I'd leave up to people more knowledgeable in the subject.