IBTL too!
Anyway, the standard theory would be that there wasn't enough evidence to win the conviction. Doesn't mean he was innocent, just that the prosecution didn't think they'd win their case so didn't want to waste more money going after it. Usually they can just win by wearing down the other party as the other party doesn't have the money to fight it. In this case they'd have a fight. Probably something else came to light to weaken their case.
I've been on a jury before where I thought the guy was guilty, but there wasn't enough evidence to convict and the rest of the jurors gave him the benefit of the doubt.