You forgot some key info:
The sole survivor, Josh Ryen, had told a social worker in the emergency room that the murders were committed by 3 or 4 white men. Josh spelled his message by pointing at letters on a clipboard as he was unable to speak, but the social worker and medical staff observed that he was lucid and could spell his name and address correctly. Judge Fletcher wrote, "Deputies misrepresented his recollections and gradually shaped his testimony so that it was consistent with the prosecution's theory that there was only one killer." Jurors, however, said they disregarded Ryen's testimony because they believed he was confused and traumatized.
On June 9, a woman named Diana Roper called the Sheriff’s Department to tell them that her boyfriend, Lee Furrow, had come home in the early hours on the night of June 4. He arrived in an unfamiliar station wagon with some people who stayed in the car. He changed out of his overalls, which he left on the floor of a closet. He was not wearing a t-shirt that he had been wearing earlier in the day. He left the house after about five minutes and did not return. [Roper and her father] both concluded that the overalls were spattered with blood. Roper turned the overalls over to the Sheriff’s Department and told the deputy that she thought Furrow was involved in the murders. Roper later provided an affidavit stating that a bloody t-shirt found beside the road leading from the murder house had been Furrow’s. It was a Fruit-of-the-Loom t-shirt with a breast pocket. Roper stated that she recognized it because she had bought it for him. She also stated that a bloody hatchet found beside the road matched a hatchet that was now missing from her garage. [...] The Sheriff’s Department never tested the overalls for blood, never turned them over to Cooper or his lawyers, and threw them away in a dumpster on the day of Cooper’s arraignment."
Furrow had been released from state prison a year earlier. He had been part of a murderous gang, but had been given a short sentence in return for turning state’s evidence against the leader of the gang. The leader was sentenced to death. Furrow told friends that while he was part of the gang he killed a girl, cut up her body, and thrown her body parts into the Kern River."
We know you can implant false memories, especially in kids. If the cops decided Cooper was guilty it would have been easy to coach him into giving the testimony above.
I'm not saying he is innocent or guilty, just that there appears to be some doubt, much of it due to police sloppiness and misconduct.