No, that wasn't really it. Their enclosure is surrounded by thick trees and closed fences. There's only a few spots where you can see through it. Then there's the glass panels, which are disgustingly filthy and an absolute horror to photograph through, so you're not left with a lot of options.
Then there's the problem of the light sensibility of the lens. This is a big lens and it needs a lot of light...which isn't there So yup...difficult