The parts that can be feudalised already are. The big companies make the machines and sell them. Big companies make the software. What is left is the physical efforts of finding a location, putting the machine in place, and then tending it regularly. There isn't much to be gained by adding a layer of management over the top of that, so big players can't really skim off the top by good management making things better.
Also big companies have too much to lose to fraud. The route operators are alone with big piles of cash. It is very easy for someone who knows what they are doing to "discover" a broken a machine (read they break it) and it was dispensing the J row without payment, then take the extra cash.
All in all it is best to leave route operations to little players.
I would imagine locations are somewhat "sticky". Your local auto repair shop isn't interested in booting their vendor for another one that charges 65c for a soda instead of 75c, as long as they're not getting bad service. And they're not interested in the conflict and confusion of allowing a 2nd operator to put a competing machine right next to it at a slightly lower price.
I dunno, I'm fairly sure Coke machines are all owned / franchised by Coca Cola; after all, you can't just use their brand on the outside of your machine. I think.
Also surprised it hasn't been feudalised somehow. Some company owning all the machines and then uber-ing out the hard work of siting them and restocking them. Gamifying the process of monitoring, "your fleet", the "ching" on your phone when someone buys a bottle of water or candy bar. Special deals for site owners. Branded supplies. An app for customers to talk to the vending machine. Loyalty points. A snack radar. Etc.
I think it's great the market is mostly independent operators.