Because the US stopped being American and became Western again:
By the early 1990s, cereal
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hit $14 billion a year in sales. Cereal
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was in 90% of American households. And
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the average American ate an astounding
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160 bowls a year of the stuff.

today,
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only 12% of American households eat any
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cereal at all.

American comments:
Which also included a glass of oj, toast, muffins, and many other foods 
I'm from Grand Rapids Michigan and I remember those cereal tours. You would get a bunch of free boxes at the end.
The best part of the cereals of 60s 70s and 80s were the prizes packed inside.
When Saturday morning cartoons fell off, so did the cereal industry
Western comments:
I stopped eating cereal because I don’t consider it real food.
It wasn't cereal alone; it was the whole carb-centered, low-fat/fat-free dietary hypothesis it was a part of, represented by the USDA's Food Pyramid. Bad science produced decades of misinformation and counterproductive dietary habits, of which the cereal and OJ breakfast was only one.
we know carbs are bad
food is made of animals