The most likely explanation is the prior to perhaps 200 years ago the differentials were different. (Also having lots of children may not result in a lot of descendents, so is an insufficient measure). This would be due to the fact that in an era before universal rights and universal economic security there were reproductive consequences to intelligence (or lack of).

So there is no evolutionary pressure. Evolution is a (brutal) system of unfairness that represents everything that modern progressive values work against. This is something that will need reconciling (or many technologists will hope for genetic engineering I guess) or in the long term will not be sustainable.

Consider your children are on average genetically inferior to you. This is mathematically true. A society which uniformly has 2 children per person + 100% survival will regress. In fact it may take something closer to mediaval or even prehistoric reproductive outcomes to have positive evolutionary pressure. This could be many people having 4-5 children, with many other people who are 'unfit' not having children at all (either remaining childless or dying early).

The subsequent improvements in education,environment and nutrition have likely provided a dividend that masked a fall in underlying genetic intelligence. Naturally as those gains have been fully realised the underlying trend will reassert itself.