I would say that there were two reasons why the jury rejected all of the DNA evidence and acquitted Simpson. One of the reasons has to do with DNA while the other does not.
The reason that has to do with DNA is the fact that there was so much expert testimony for both sides about the credibility of the DNA evidence. When juries that are made up of laypeople are exposed to complicated scientific testimony, they may well become confused or bewildered. If there are scientists testifying for both sides, they may come to doubt that the prosecution’s version of the evidence is true beyond a reasonable doubt. I believe this happened in the Simpson case.
The second reason has to do with the fact that DNA evidence is only as good as the credibility of those who collected and processed it. In this case, the clearly racist opinions of Detective Mark Fuhrman made it possible for the jury to believe that Fuhrman had planted the evidence that implicated Simpson. The jury did not have to disbelieve the DNA evidence. Instead, they only had to have a reasonable doubt that Fuhrman might have planted the evidence to frame Simpson. I believe that this, too, led to the creation of a reasonable doubt in the minds of the jury.
Thus, the acquittal was probably due partly to confusion over the complicated scientific testimony and partly due to the jury’s doubts as to whether the evidence had been found and handled in an honest way.