Unfortunately I've never had the honor of serving on jury duty. But the US is a nation of laws and citizens. When someone is accused of breaking the law, if the DA deems there is enough evidence to go through the expense of putting that person on trial, it will go forward. At that point, all the DA can do is make sure the evidence is as airtight as possible. Sometimes witnesses are not as forthcoming as the DA thought they would be. Sometimes their testimony is not very compelling to the jury. Sometimes the DA over-reaches and tries for a conviction on a charge for which there is not a clear case. For many reasons, cases which to an outsider look airtight don't end up that way when the jury goes behind closed doors and ponders the evidence WHICH THEY HAVE OBTAINED DURING THE TRIAL. And sometimes, as in the OJ case, you get jurors who are looking for political payback, so they won't convict someone who according to the evidence given in the trial seemed clearly guilty.
It can be a messy situation, but that's the way of 'Justice' in this country of free citizens. It is not a perfect system because it depends on humans, who make misakes. There are a lot of guilty folks walking the streets today because the Justice system failed in some way or the other. It shouldn't cheapen the system because of those mistakes, it should make people more interested in serving on jury duty when they are called so that they can be sure of at least one person who really cares that justice is done. Frankly I think it is a miracle that the system works as well as it does, and I hope that if I ever find myself caught up in it, there will be educated citizens who are serving on the jury because they care about it.
And sometimes someone in position of high power prevents any of this from happening at all. As is the case with the Bush adminstration.