Human societies can be controlled according to what the majority believe to be true. The beneficiaries of the popular belief are given opportunity for unfettered power and related control which can be employed to both maintain and entrench the popular belief.
But broad-based intelligence in a society can become a threat to those with the inherited primeval urge for power and control. Since the retention of power requires control of opposition then the intelligence must also be controlled. Education must be controlled. The internet must be controlled.
The above logic may also provide some input to explain why different societies have developed intelligence at differing pace or not at all, as DNA analysis can show.
Susceptibility to indoctrination and resulting obedience or radicalization can also be indicated. This is because we are animals and have been channelled into groupings of type of sub-species by the same mechanisms that affect all reproducing life forms.
Culture is superstition wrapped up in indoctrination. Culling intelligence makes the retention of power and control much easier.
Conspiracy Theories that Proved to be True