The problem is, there's often not a lot of evidence either way. Police can't do nothing and it isn't their responsibility to determine guilt or innocence, their job is to gather evidence based on an allegation and then the prosecutors determine if there's enough evidence to pursue a prosecution.
I think police often try to make "evidence" look like more than it is but prosecutors can also take the blame for pursuing cases that lack evidence. I'm not saying that's necessarily the case here, on both occasions at least some of the jurors were convinced of their guilt.