Just a cursory look at this case reveals the prosecution didn't use DNA evidence because even the DNA results they had proved little. For instance, the victim's DNA was found on Mitchell's pants. This could have been through innocent contamination though there was no way to prove it ot there during the murders.
It seems like some people want to ignore that her DNA was found on him but then to claim any DNA found on her items has to be from the killer. The reality is that the same contamination possibility exists across the board, it was her sisters shirt so not surprising the DNA of her sister's boyfriend was found on the shirt. Only DNA from semen inside the victim or blood based DNA would be highly reliable.
Mitchell was seen near the seen of the crime around the time of the murder, his own brother busted his alibi and he found the body though he should not have been able to see it from where he supposedly located it. I doesn't look good for him and then when you factor in some other things it looks like he is the killer.