The demographics of the West have also changed over the past hundred years. Members of these demographics consistently score lower on IQ tests so that’s a contributing factor to the lower average IQ of the West.
People also don’t critically think as much now as they once did. They have the TV tell them what to think, and calculators and computers to give them instant answers.
It’s so easy to figure out, but politically incorrect to name! It’s nothing more or less than the unrestricted immigration from the low IQ countries into the West that accounts for this.