The DNA "evidence" was thrown out as worthless. Not only was it almost certainly the result of contamination (the police didn't change their gloves according to procedure), it was present in minimal trace amounts far below those accepted as significant under FBI and various other international standards.

What's more, Guede was identified as the murderer by the fact that his DNA was all over the murder scene, and Knox's & Sollecito's was essentially nowhere. The police took the alleged "murder weapon" from Sollecito's kitchen solely because it was a big knife. There was trace DNA on it but no trace of blood - not even, it seems, at that substandard level.