Their returns (margins are irrelevant) are usually lower than grocery stores. Large retailers will turn over their inventory tens of times a year, dollar stores won't so the returns are typically lower.

For some reason, left-wing journalists turn into law of one price zealots when confronted with this issue. The reality is that these locations have low-volume and stores everywhere are relatively expensive to run now. For some reason, journalists get angry at the company rather than people who control how much it costs stores to operate. I mean local governments in the US had no problem accepting Dollar General's sales tax from their poor constituents shrug probably more than the corporation is making from the store.

I live in the UK and there is a store like this, Co-Op. The Guardian finds it easier to blame evil foreign corporations because the Co-Op has much higher prices but is a non-profit so the narrative of the evil corporation crumbles.