I think I'm right in saying they don't do well in grassy paddocks due to parasitic infection- so are often kept on bare ground like this. It doesn't look nice- IMO they aren't a good zoo exhibit anyway. I suspect some places keep them mainly for the bonus of being able to utilise them at Christmas etc.
It was a very short grassed paddock, but I'm guessing the deer have turned it to mud. Probably not intentionally kept like this as it doesn't look good.
