White college educated males were considered the default in a lot of early and mid 20th century experimental psychology due to the availability of test subjects and biases of people running the experiments. The definition of neurotypical behavior came from a subset of the population.

ADD and ADHD are known to hinder progression through the education system. So, the cohort of people on that part of the spectrum were underrepresented. The bounds for being normal were skewed.

Also, medical intervention is typically only looked at when the mental condition causes a reduction of quality of life. Over the past generation, the need for a formal collegiate education has become greater in developed countries due to shifts in economics. A kid which would be considered energetic and absent minded a few generations ago would have a far greater chance in being economically successful compared to now.