There actually is enough evidence to prove his guilt - but you have to assume that the evidence found has not been tampered with. There was no discernable evidence proving that the police had tampered with the evidence, even though there were some very questionable actions taken and obvious evidence like the key that showed up afterwards...
If you were a juror on this trial and had not heard of Steven Avery or known anything about him or this case beforehand, you would probably have came to the conclusion that he was guilty. Another problem with the trial was that many of the jurors came in and were going to push for a guilty verdict no matter what evidence came to light, they were all from the area and all too susceptible to the DA's persuasion. They spent enough money on the case, the least they could have done was flown in jurors from another party of the state.