For US products, scale and agricultural policy (strongly preferring bulk grain, sugar, and processed-food production) has a large part to play. Another huge element is advertising, particularly children's advertising, of high-sugar cereals.

I'm wondering if part of the decline of the processed-cereal market isn't due to the general decline in television watching and cord-cutting.

Flipside is that similar advertising is likely to migrate to games and child-focused online video. If it hasn't already.

(I'm not part of the audience of either, nor would my extensive ad-blocking allow me to see what's being targeted to such content.)