Answer is pretty simple by the way,
Ultraprocessed food was a relatively new thing to the world and a few people got hooked on the addiction to it really hard, but also food was cheap, so there was a rapid binge on it. Everyone else just did a more slow addiction and gained slower.
Food is now more expensive and its not novel anymore either so you dont see the rapid, stuff themselves big, types except when they happen.
You want to see this come back food prices would need to drop fast while the rest of the world being expensive, also add like 3 decades of sugar/calorie reduction back to the food.