We don't know what information they have so it is not possible for us to say specifically why they have ruled the McCanns out as suspects. They may have concrete proofs; they may have information that gives them compelling reasons to believe the McCanns are innocent. We don't know.
All we know is that they have said that the McCanns are not suspects. They are not obliged, and perhaps not even able, to tell us exactly why.
We cannot assume that because we have not been given details that SY have got it wrong. That is not a reasonable interpretation of the situation according what we know of it.
I think your post is misleading
The suggestion that Scotland Yard have 'ruled out' the McCanns as suspects implies that they ( the McCanns )
were, at some point, suspected of involvement by Scotland Yard and that subsequent investigation and evidence had
proven that they were not
There is a world of difference between being ( currently ) not suspect and being 'ruled out' as suspects
Scotland Yard have
never said the McCanns have been
ruled outWith regard to Scotland Yard being 'not obliged' to speak of their reasons for 'ruling out' the McCanns as suspects I do not follow the logic
If they have uncovered irrefutable evidence that the McCanns were not involved in their child's dissappearance then what possible reason could Scotland Yard have for not saying so ?
I mean, they did it with regard to the man Jane Tanner saw, didn't they ?
They said that he was no longer a suspect because evidence had been found that 'ruled him out'
Why havn't they done the same thing with the McCanns and simply announced that evidence has been found that 'rules them out' ?