Did you even read the story? They intentionally withheld this evidence because it would have helped the defense.

>Some of the withheld evidence included videos from surveillance footage, memos from investigators, communications between prosecutors and cellphones from people arrested after Mr. Franco identified them as drug dealers, Mr. Tanner said in an interview. He described the evidence as “potentially exculpatory.”

>Mr. Tanner said that prosecutors in court blamed their failure to turn over evidence on “gross negligence,” but said that he did not trust that their actions had not been willful.

They did it to win the case, not lose. And when they got caught, they said "oops, it was an accident".