That said, judges have a lot of latitude to allow evidence of prior conviction if they feel it is in aid of a fair trial. They wield this power in order to keep all parties honest and unbiased as best they can. In this case - rightly or wrongly - the judge has decided that knowledge of the outcome of the first case would prejudice the second case, especially due to the emotional nature of the allegations. That's why the order was made.
While I have nothing but contempt for Cardinal Pell, who represents pretty much everything I dislike about the church, he has as much right to a fair and unbiased trial as any other person. Months of media saturation might indeed prejudice that, so I reluctantly support the intent of this kind of power, appropriately limited and temporary. It certainly is more complicated in the age of global media, however, and I really don't know what the answer is, if there even is one.