Because bark looks really sweet/smells good/holds humidity well but is expensive and a pain to keep clean.
Also sometimes it's too humid, and some snakes need humid zones and dry zones.
I just keep the snakes all on aspen in unusually large tubs, then have a smaller tub inside it with a lid on it and a hole cut in the side with moist substrate like bark, bed-a-beast, or peat moss.
Some snakes like moist hides, but pretty much all will use it during shed.