I think they didn't import a bull because they wanted free contact will all their cows, Australia Zoo is a bit old-fashioned and out of touch in that way.
That makes sense, in addition to lacking the infrastructure to house bulls. In addition to no contact facilities, they’d require stronger reinforced barns etc.
If they were serious about breeding (which may have their doubts), having a bull on site could ultimately be cheaper than repeated cycles of AI, which may result in bull calves anyway.