In our court system (which is far from perfect, but is still arguably the most fair in the world), you are presumed innocent until proven guilty beyond reasonable doubt. Many people in Chicago have seemed to agree that the prosecution did not do a very good job of making their case, but then turned around and act like he should have still been convicted!
A jury only gets a very narrow view of the case. They only hear the arguments and defenses that the judge allows to be presented to them. Based on that limited scope of information, if the prosecution can't reasonably prove guilt to the jury the first time, after over a year-and-a-half of work (at taxpayer dime), why should there be a retrial? None of us was in the room with the jury, and none of us really knows who was taking what sides or what information they had to work with -- how do we really know that the lone holdout wasn't the only one thinking clearly and giving honest, unbiased thought to both sides of the case presented?
Both the defense and the prosecution agreed to the jury members -- if either side has a problem with how those agreed-upon jury members then ruled, that's not a reason to redo the case at the taxpayer's expense (unless there were clear failings of the jury to do their job properly). Yes, justice should be served, but in our system, if the jury can't find a person guilty, then justice has been served. At least in terms of this system -- obviously only God truly knows the contents of our hearts (and holds us accountable for it).