Its a cultural thing I am sure. As an American I will say that prepared food served from a vending machine is going to be associated with low quality and possible poor hygiene. I'd equate it with food from a gas station store or roach coach (mobile canteen)- food prepared with little care or quality, destined to be sold for as little as possible while still being profitable. Stale bread, wilted vegetables, low quality meats, cheese, etc, sloppy prep. Who cares, ship it.
On the other hand, I can see food vendors in Japan guarding their reputation with attention to their craft ensuring quality.