The main reason - imo - is simple demographics.
It is a fact that the wealthier you are?
Generally - the more educated and more intelligent your offspring will be.
A lack of proper diet, medical care and education can effect the IQ of your offspring.
That is why poorer countries test lower for IQ.
It is not that racial bullshit that some people cite.
It is that they are poorer and have less access to healthcare/education.
https://worldpopulationreview.com/country-rankings/average-iq-by-country
In the last 60 years?
The fertility rate of western women has dropped...STRONGLY.
Less so in less developed countries.
https://ourworldindata.org/fertility-rate
https://en.wikipedia.org/wiki/List_of_countries_by_past_fertility_rate
And even in developed countries?
Poorer people have higher fertility rates.
https://www.statista.com/statistics/562541/birth-rate-by-poverty-status-in-the-us/
100 years ago, many of the poor's children died.
Now, with better healthcare, they survive.
Bottom line is middle class and up women (with higher IQ's) are having less babies.
And poorer mothers (around the world) are producing most of the children.
The latter are - with exceptions - of lower IQ's.
Thus...the world IQ level is dropping.
And why since the mid-1970's?
Because starting in the late 1950's/early 1960's?
That is when wealthier, Western women started entering the work force.
And stopping being 'baby machines'.
It's nothing strange.
Just simple demographics.