Because local prejudice can provide bias, jurors can be influenced by a lawyer's courtroom performance, most juries aren't actually a random sample of the population, long trials usually create hasty verdicts, most jurors don’t have a background in law, and jurors do not need to reason their decision.
Not to say that there aren't pro's to trial-by-jury, but not every Westerner is convinced that trial-by-jury is better than many of the alternatives, and that doesn't make a Westerner any less of a "Westerner."