What would be possible reasons for someone with DNA evidence against them to be aquitted of murder and rape?

I haven't been able to find out why this guy was aquitted? He was tried and found guilty and was in prison for some time, but is now free. He went to my high school although I didn't know him. My best friend was friends with him and to this day doesn't believe he could be capable of such things (although he does have past convictions of rape among other crimes).

Here's a bit of the story:

DNA leads to arrest in 1991 killing

By Valerie Schremp Hahn

ST. LOUIS POST-DISPATCH

Friday, Dec. 15 2006

Almost 16 years ago, a highway worker clearing ice from Highway W in Lincoln County found the partially-clothed body of 18-year old Stephanie Hogland face-down in a ditch. She had been bludgeoned to death.

Today, investigators announced a break and arrest in the case, saying it is the oldest case in the state solved by a DNA match.

Michael Edward Dowell, 44, an unemployed painter who lived in the 9700 block of Newton Drive in Ferguson, was charged with first-degree murder, armed criminal action, and forcible rape. He had submitted a DNA sample on Oct. 30 as part of his probation for an unrelated assault case in St. Louis. He remains in custody, with no bond set.

On Nov. 30, the Missouri State Highway Patrol Crime Lab reported it had a match. Not only is the case the oldest one in the state solved by DNA, the DNA sample taken from Hogland�s underwear in 1991 was also the first sample to be submitted to the Missouri State Highway Patrol Crime Lab, Highway Patrol spokesperson Julie Scerine said.

"Never give up," said Maryland Heights Police Captain Bill Carson, commander of the Major Case Squad of Greater St. Louis. "There are cases that are going to be solved in the future because of technology like this."

Hogland�s parents, Gene and Gayle Hogland, and her two younger sisters stood by at the Lincoln County Justice Center today as investigators released more details about the case.

Hogland, of Wentzville, was last seen in the early morning of Jan. 6, 1991, after attending a wedding reception with friends in Old Monroe. At some point on her way home, she got a flat tire, and she pulled over her 1977 white Pontiac Firebird in a driveway along Highway W north of Highway 47.

The owner of a nearby house found the car later that morning, called the sheriff�s department, but they found no sign of Hogland. More than a day passed, and the highway worker found her body about 15 miles away from her car in a ditch alongside Highway W north of Highway B. She had been raped and beaten in the head. An autopsy later revealed she died of closed head trauma.

Investigators worked hundreds of hours on the case, interviewing hundreds of people. They did not interview or suspect Dowell, who lived off Highway Y east of where the car was found.

"This appears to be a crime of opportunity," explained Carson. "It would appear she had car problems and the wrong person stopped to help her."

Police went to Dowell�s home Thursday afternoon and interviewed him at the sheriff�s department. Detectives took Dowell to the spot where Hogland�s body was found, but he denied any involvement in the case. They then arrested him. Even after they showed him pictures of Hogland, he denied knowing or seeing her.

Lincoln County Prosecutor G. John Richards said police have more evidence against Dowell than DNA evidence, but would not elaborate. He said prosecutors would consider seeking the death penalty against Dowell because of the brutality of the crime.