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My guess is that it is for the nurses benefit. If they have to change one on someone that can not move around much than all they have to do is untie it and slip it off.

You can turn it around , or put another one on backwards if you are walking about out of your room. Most people wear a robe when not in their room. You do not see many bare buttocks in the hallways. Nurses and Physicians need access . Even when the gown is removed, the "Private parts" are covered during an exam. ( unless that is what we are examining)

They are easy to put on and take off. Plus, in an emergency you don't have to cut a gown off to treat or shock the patient. One size fits most as they are tied in the back. They don't do a lot for modesty.

It makes it a lot easier to remove it from someone who is bed-ridden or in a wheelchair. If it opened in the front and was closed in the back, one would have to turn the patient around on their front to pull it off.

they make for easy clean-up. if a patient is unable to stand, one can just untie the gown and pull it off (no need to go up and over the head).