Energy efficiency. If a hotel has 100 rooms, the running costs of running 100 fridges 24/365 are large, especially in climates where air conditioning is needed, since the AC has to work harder to overcome the heat given off by the fridge. They also represent a big capital cost to provide in the first place, and maintain and replace them as they fail. And they take time to clean and restock, and make checkout more complicated.
But you can still offer "minibar menu" - with options like beer and salty snacks, or fruit and soft drinks - and when you phoned room service, they would bring your chosen stock.
Not a bad decision as the guestroom refrigerator is often the noisiest thing in the room, aside from a room’s air conditioning, and has been known to negatively affect the guest’s sleep experience based on how hard the refrigerator is working to keep itself cool. In some situations, guests unplug the machines for a night’s sleep, leaving them unplugged, and often overlooked by housekeeping.
The average guest will open the room’s refrigerator once in a 48-hour period.
Another option is to offer refrigerators as on-call items but not enough for every room. So, the guest who order first this complementary service will be first served. Once the hotel runs out of refrigerators, they are out.