Unfortunately juries are far more fallible than most would like to believe.
Indeed, there is a reason why many want to exclude eye-witness testimony as unreliable. In many states, it is a matter of law that single eye-witness testimony is not sufficient to convict somebody of anything.
But in a day-and-age where crime is thought to be on the rise and citizens are seen as doing their duties by getting more rapists off the streets, this outcome is not entirely unpredictable. 25 years ago (and even now), there are a lot of people out there who assume that if you are black, if you didn't commit the crime you are charged of, you did something worth the same penalty.
Jurors are notorious for hearing what they want to hear.
The prosecutor paints a terrible picture filled with horrid but (totally unsubstantiated) detail, produces a sympathetic withness who dramatically points a finger at the dregs of society (actually two men both in suits and ties listening to her lies), then he stands up and says "YOU MUST CONVICT". The defense can only stand up and say "These are nice guys and though I can't prove they didn't do it, only that you don't have enough evidence to convict."
The affirmative directive wins. Everybody goes home feeling they have done their jobs. Everybody except the two falsely accused who are left wondering what the hell just happened.
Should we be surprised? I don't think so. Should we work very hard to make sure it doesn't happen again?