I wonder, how did police discover he had a 'neighbour relationship' with the Jessop family? They didn't interview the Jessop's before informing them the case was solved, maybe they contacted his ex-wife? Or perhaps he was in the file, but appeared to have an alibi.
Just speculation but it could be he was overlooked because he was such a trusted, utterly normal friend of the family, the spouse of a good work friend, with children who were playmates of Christine, a frequent host and a guest...how could it be conceived that such a trusted person would inflict torture and murder on the little girl and the family? IMO, that is a profoundly shocking betrayal of every level of human decency. IMO, family and even LE, look for someone who fits the profile of 'deviant', someone you would never trust to begin with.
JMO