It is an interesting puzzle. This was a secular rise in cognitive test scores well documented in a number of countries during the 20th century. It has stopped and even reversed in the last few decades. There seem to be several pausible ideas out there

One is that social changes have had the effect of "training" people for cognitive tests: more magazines, radio, chatter everywhere, advertising, etc. Hard idea to test. I do fieldwork in Southern Africa. Forty years ago there were no radios in the backcountry, no books, no magazines. Today radio, newspapers, magazines are everywhere. I expect that this changes people a lot but I have no evidence.

Flynn himself thinks nutrition got better but the data are not clear about that. I would favor as an explanation vaccination and antibiotics. Infectious disease and the inflammation associated with it does seem to damage people (Caleb Finch, Eileen Crimmins, others). We have cut the intensity of childhood insults way down everywhere.

My two cents........