- Other countries are catching up. One measure of the US "falling behind" in education is that other countries are performing much better on all sorts of exams. While the US is not putting a lot of investment in math and science education, countries like China, Russia, and the Czech republic are churning out math Olympians.
- Because more people are educated, the standards have become higher. Now that more people are getting educated, people are no longer impressed when someone else knows the entire Western canon**. But in seriousness, people were considered well-educated before when they knew the basics in a lot of things and then did some medium-depth study of classics or something like that. It takes a lot more to be impressively well-educated today. My evidence for this point is that education at the top (at places like Harvard, at least) certainly does not seem to be getting worse. When I talk to my professors who are over 60 and went to Harvard as undergraduates, they seemed to have a lot less pressure and more free time. (But does pressure = better education?)
- When we think of education "back then," we were taking a median or mean over education of only a portion of the population. Since then, education has spread to all, causing the overall quality education to become worse. (This is similar to saying that the quality of asymmetrical dresses was much higher "back then," two months ago when they were only on the runways. Of course they would be; there is a reason why certain designers design for Chanel and others design for Target.)
*I mean absolutely worse, which is the common usage of the term. I mean, when your favorite Olympic runner's world record gets broken by someone else, you don't go around bemoaning the fact that s/he is getting worse by the day.
** This is a facetious point. People are no longer impressed because there is no longer a fixed Western canon and also people have come to devalue education to the degree that familiarity with literature is no longer impressive to most.