If we assume it's intentional I would guess the problem is the majority of US places are either too population dense or not dense enough. There's a finite amount of parcels that can be stored by these sort of systems; if the city is too dense (and too many people opt to use the system) it will become a gamble on if there's a place for your parcel there and the value of this system is you don't need to organise your day around deliveries.

I suspect the reason BufferBox can work is because they're not a delivery company, so they have no obligation to try and fit it in to their current business model. Although then a pre-existing delivery company could have always gone with the BufferBox model... so, maybe it's just not that profitable in big places like the US?