Cellular modems and data plans can be expensive and unreliable. If you can do without, at the cost of very rarely being out of sync (like in your scenario), it

Cellular modems and data plans can be expensive and unreliable. If you can do without, at the cost of very rarely being out of sync (like in your scenario), it can be considered an interesting tradeoff. Plus, even if they were equipped with cellular modems, they probably have a business requirement that the locker should be usable even when the cellular connection goes offline, so you have to design your protocol to be Bluetooth-first.