Because with a trial by jury, a prosecutor must convince TWELVE people beyond any reasonable doubt, whereas, should you choose to have the Judge try the case, it is only ONE. If I had to have a trial, I would choose trial by jury every time. For some reason almost every case I have seen tried solely by the "Judge", it usually goes the prosecution's way. I believe 12 jurors are much more fair than 1.
From what I've read, this is how it started out and obviously time have moved on by now, but..This system was created in England back when all the land was owned by lords and ladies in the modern more logical view It's better to get judged by people of your peerage than somebody out of your way of life that might not have any idea how your life situation brought you round to committing the crime in question. In other words, who would you rather choose your punishment or sentence, somebody in your village or somebody who owns your village?
With a jury the accused has a better chance of a fair trial as opposed to a biased individual. What it really means is good attorneys choose the most ignorant uninformed gullible people the believe they can sway to their ( the clients) cause. It's not about the truth it's about what a lawyer can convince a jury to believe .