Furthermore, you have a point about customer loyalty and classism. Perhaps, Dollar Tree is seen as low tier for some people. 

Moreover, for some, it could simply be convenience. Maybe some people don’t want to go to several stores to get items. But that’s also off-putting because in some places Walmart and Dollar Tree are literally in the same area. In one area I frequent, they are in the same shopping plaza. 

Let me also add a quick story: At a particular Dollar Tree, I asked the manager why they didn’t sell more food items. She said they used to. There’s a Winn Dixie in the same shopping plaza. Winn Dixie complained and said Dollar Tree was taking business away from them. The owners of the shopping plaza told Dollar Tree they couldn’t sell many food items because they’re competing with the grocery store next door. Technically, Dollar Tree isn’t a grocery store. So, now this Dollar Tree has limited food items for sale. 

 Despite some of the the simplistic answers I’ve seen, it doesn’t make sense to me not to acknowledge the class aspect. I think there’s cultural inertia or societal pressures involved that may be driving the decision to shop in stores that are both higher in cost and lower in quality.