Cats
Because all of that promotion is funded somehow. It doesn't come out of profit margins. It is eventually (and it may take time if promotion was introduced overnight) absorbed into the base price.
For families on a tight budget, buying two tubs of formula (for example) might be out of their abilities. So they can't access the 3 for 2 discount, but they do experience the higher underlying price.
Or, as the base price becomes higher, the acceptable price to charge in the corner shop becomes higher. Which is the only shop they can afford to travel to do their shopping when they run out mid-week.
Or there is a big promotion on at the Tesco down the road, but they can't afford the bus fare. So whilst all the middle class mums stock up in Tesco, they are stuck with the Tesco metro/Co-Op/Asda not running the promotion.
Or companies decide to do one off 'discount week' events where all the middle classes buy 10 tubs, but their weekly budget still only stretches to one.
Or the company decides it will spend a lot of time running X% off promotions (the DFS approach to pricing if you will). They have to have certain price reference setting periods each year though. So instead of the formula being 75p off like it normally is, one day mum/dad goes in and it's full price. And that 75p means that something else in the basket has to go back that week.
Poor people actually frequently lose out from the effects of price promotions. They just can't access them the same was as those with more leeway in their budget and more travel options. That applies to discounts on anything branded. The problem is, formula a) has no own brand basics to fall back on, and b) is vital. You can decide to live without Coke/Kelloggs Corn Flakes/Birds Eye peas (if you imagine for a second a world where only the brand is available). You can't decide to do without formula for a week.