The reason of course is that the law places the burden of proving someone guilty beyond a reasonable doubt on the government. You don't have to prove your innocence. (like you could even do that). The person accused has no burden of proof at all. In fact the trial starts with one fact absolutely established, that the defendant is presumed "not guilty". The government must prove guilt to the highest standard the law allows and if they don't the jury has no choice but to find the defendant "not guilty". A criminal trial is not about what you as the defendant can prove. It is about what the government can prove.
It has been my experience that I have won many more trials when I put on no witnesses or evidence. Instead I shot holes in the government's case and pointed out how weak or how little evidence they produced. Often by putting on witnesses and evidence for the defense, the jury gets confused that the trial is not a battle of who has the most or best evidence. They think, "well we believed that the state's evidence was better or they had more evidence than the defense". We will lose that battle every time. When no evidence is produced by the accused, the law about who bears the burden of proof is underlined for the jury. The court will tell the jury that the law does not impose any burden on the defendant to produce any testimony or evidence. The judge instructs the jury that they must accept that as the law before they begin their deliberations.
So why is this distinction not well known? If you listen to the news coverage of a trial, you will hear them talk of innocence. They will say a jury found someone "innocent" in court today. No they did not! They found someone "not guilty". If you are concerned about innocence, then see a priest, rabbi, or clergyman. If you have been charged by the government with a crime and they seek to take away your liberty, you should be concerned about being found "Not Guilty" and see a criminal defense lawyer!