Comment by kelnos

For Japan I expect it's also a matter of population/crowd density in the cities. There are tons of staffed convenience stores (7-Eleven, Family Mart, Lawson), but even with a high density of stores, they're often fairly crowded.

Having lots of vending machines even for simple things like bottled water and soft drinks reduces the pressure on the convenience stores quite a bit. More advanced vending machines with other products helps even more.