Building/fire codes are a big factor. When you have enclosed spaces with doors that shut tightly then a variety of regulations come into play, whereas with a loosely-fitting partition with a gap underneath they do not. If push came to shove you could kick most partitions and they would have enough flex to pop the door open, or you could crawl out on the floor.

Apart from that - the ability to see if the stall is occupied without tugging on the door and perhaps causing an awkward encounter, making it more difficult to smoke or shoot up stealthily, and just simple cost/ease of installation.