One could also guess that they don't have as much confidence in the accuracy of their evidence-gathering method as they'd like. If they have to show how they gathered that evidence, and the defense is able to introduce enough doubt that their method targeted the right person, it would torpedo all related cases, and hurt the prosecution's credibility in future cases.

We're taking it as a given that this guy (and the others) actually committed the crime, but the prosecution cannot actually prove that without allowing us to verify that the evidence gathered actually comes from a place of fact.