A trip to a hypermarket is a comparatively big deal - you typically have to travel further and you are also going to spend more money. So to the extent budgets are tight, people will make do with more frequent local trips, accepting the price hit that follows. (Same reasoning applies to time budgets - make 10 20-minute trips instead of one 3-hour trip).
The other issue (at least in the US) is Amazon and other online retailers. Suddenly people can get a decent price without having to make the trek to a hypermarket.
The final point is competition from the membership-only stores - they carry a much smaller inventory and hence can be more aggressive on pricing.
(Don't know how much of this applies to developing nations).