Because pigs (wild or domestic) make a mess of anything you put them in. I agree this is ugly (the new barrier makes it far worse), but I don't know how to maintain foliage in a pig exhibit without making it unrealistically large. Originally this exhibit held raccoons and it was a beautiful exhibit back then.
@Arizona Docent I agree that pigs would destroy any and all vegetation in the exhibit, but would it kill them to at least put mulch or something in there at least?
Maybe mulch would work? Yes I think that is a kiddie pool on far left. Not seen are a pair of sleeping pigs on far right in front half of exhibit. From what I could tell, I think at least one of the sleeping pair (perhaps both) was a male and the one in back shown here is a male, so that is likely reason for partition.