Because of people like Bob Barr. He's a former federal prosecutor and congressman from GA, who has publicly stated his support for the DP, and one of the original authors of the Anti-Terrorism and Effective Death Penalty Act of 1996, which he claims was misinterpreted by the lower courts who refused to hear Davis' petition for a writ of habeas corpus. Nor has the court heard witnesses whose testimonies have changed, only their written affidavits.
and it's not "no DNA", it's No Physical Evidence, zero, zip.