Why do prosecutor try to block DNA testing which can prove innocence of inmates?

Today a man was released by DNA evidence. He served 27 years for a crime he never committed. Prosecutors tried very hard to block DNA testing of evidence. They kept blocking DNA testing since 2005. If they did not try to block, the man would have been released in 2005 instead of today. Those prosecutors wrote letters to governor urging him to disallow DNA testing. They said that the man himself confessed to doing the crime and that's ultimate proof. Now the man says he was forced and pressured to confess and was threatened with death penalty if he didn't confess.