With the amount of access to electronic media these days (take a footy forum for example) finding a jury now without this knowledge will be very difficult.Because someone is entitled to be judged on whether he or she committed the specific crime based on the evidence available, not on a hunch based on their past conduct. There are some situations in which evidence of past behaviour will be allowed before a court - eg where the defendant has committed similar crimes with a similar m.o.
The reason for not allowing discussion of those kinds of things before a trial is that it is up to the Court to determine whether they meet the threshold for being admitted, not to the media or public.
If someone is found guilty, then their past conduct will be relevant to working out a suitable sentence.