Unfortunately, the police and/or prosecutors were so focused on the Ramsey's that they apparently disregarded (and possibly suppressed) any evidence which didn't seem to point to them. And the news media stepped right into line with the police, at least at first, all but convicting the Ramsey's of the crime without any corroborating evidence.
I haven't paid all that close attention to the McCann case, but it's my understanding that the Portuguese police are doing much the same, focusing on the parents to the exclusion of any other evidence. If this is so then you certainly can't blame the parents for getting their message out into the open, forcing the police to look elsewhere.