It does seem quite interesting to me, that the two doctor parents of the girl would be prime suspects, when there was at least one dodgy guy like this one living in the area. Why anyone would distrust a group of doctor/parents, and trust dodgy single guys living in vans is quite confusing to me.
BIB - because of the technical way in which inquiries are conducted.
Police usually proceed on something called "theory of the case". The general idea is that if you investigate the correct theory, you will find corroborating evidence whereas incorrect theories will remain fallow.
So in this case it would appear the police (correctly) adopted 3 theories.
1. Abductor.
2. Wandering
3. Parents.
Theories 2 and 3 are statistically likely. 1 less so. Theory 2 was swiftly eliminated. Contrary to popular belief PJ did not "ignore all other evidence" but actually focussed on theory 1 because they had evidence for it. This lead quickly to the false arrest of Murat.
Facing an impasse, an english expert called Mark Harrison was called in. He conducts a complete review supported by PJ and Scotland Yard.
It is in fact Harrison who refocusses the theory on 3 factors. First Maddie is most likely dead. Second the body is likely concealed in the general area. Third, the guilty party may be in the close group (statistically)
It is Harrison who bought in the cadaver dogs, and these produced evidence pointing to the parents.
So how the PJ investigation ended up where it did is not really a mystery - they followed where the evidence led.
Police don't have the resources to trawl through hundreds of potential suspects in detail to find the guilty party.