I remember my parents visiting me and turning their noses up at my Cheerios saying (with the most disdainful tone) “We don’t eat sugared cereals.” And they pulled out their “healthy” granola cereal to eat with great superiority instead.
Later, after they left, I was in the grocery store and compared their brand to my Cheerios. The granola had twice the sugar grams per serving.
If the OPs conclusion is true, I think it’s a reflection on the target consumer.
Consumers of adult cereals like Raisin Bran or Grape Nuts are more brand loyal, having been eating a limited range of cereal brands for many years. They know what they like, they don’t stop to think about it, and they are more likely to be brand loyal whether there is a sale or not.The Coco Puffs fan, on the other hand, is likely younger and more impulsive, and there are more options for them. If not Puffs, then Pebbles, Krispies, Chocula, or ten other chocolatey options.
As for me, I’m buying my same granola every week until forever. If I see Apple Jacks on sale, I may or may not grab those too.
And Cheerios goes on sale. Once the Cheerios shelf was empty at my supermarket due to a sale, and the Cheerios rep saw me not finding it, and she hurried off and found some on an end cap. 
Christ, about two months ago they were as low as 59 cents a dozen here at Aldi. I think they’re around a buck now, unless you want the fancy stuff, where it’s closer to $3-$4 a dozen. I don’t think I’ve ever seen eggs anywhere near $7 a dozen around here. I would expect those eggs to be hand-picked and hand-polished with a picture and brief biography of the chicken(s) they came from, and what time and day each egg was individually laid.
As for the OP, the only cereal I eat is Fiber One, and that goes on sale here from time to time.
Because unhealthy foods sell more and cheaper to make giving rise to profits from sale prices when item sales are low and too many are in stock. You have to keep the merchandise moving to make way for the next batch. When sales dip, a price cut can help move those products and since they have such a high markup anyway, there’s still profits had from a price cut. Anything marketed as healthy is a market all to itself and profit margins reflect the demand.
I like Grape Nuts but I can’t eat a lot of wheat anymore. I don’t like the sugary cereals . Sugar is very addicted so that way it sell fast !