A system that is easy to manipulate can persist because those in power prefer it that way, not because nobody knows how to fix it. Reformers who tried to modernize the state's death investigations usually quit in frustration. The pliable expert was useful to the people running the system.
When an expert is paid a flat fee for each examination, the incentive is to take on as many as possible, whatever happens to quality. One examiner ended up doing most of a state's autopsies, several times the recommended load. His reports described organs that the bodies no longer had.
When the people who examine the evidence work closely with the police, the expert stops testing the police's theory and starts confirming it. Examiners found marks, sometimes invisible to the naked eye, and matched them to whoever the police already suspected. The result was that a hunch became a near-certain conviction.
Gatekeepers who must answer to voters who want tough sentences tend to doubt only one side's science. Judges let unproven methods in when they helped the prosecution. They were far more skeptical of defense experts and of research showing that false confessions are common.
Bad forensic methods do the most damage where racial and class bias already decide whose guilt seems believable. The old injustice and the new junk science feed each other. Men who might once have been lynched were instead falsely imprisoned and pursued for execution for decades.