It appears that
a) the evidence wasn't strong enough - something that looked increasingly apparent as the first trial went on. No body, no forensics
b) some of the evidence may have been from an unreliable source - you do have the problem that in these sort of cases, few of the witnesses will necesarily be pillars of their local church or well-known for their charitable activity
c) some of the police work may not have been top-grade
The police investigation continues.