A mixture of social media, lack of or standardized education, populistic media causing populistic people, and a small bit of eugenics (which again is heavily influenced by social structure: people with fewer resources will not get the same opportunities as rich people. I mean, Trump became president.).
Hard data: Uneducated people have more kids on average and at a younger age, but this should not be seen in isolation. Environment and Inheritance are vital factors in this, while some extreme people will say that Heredity is the one that counts...
And IQ (tests) can be trained. A person with a scoring of 100 can "train" to perform higher on the test over time.