Supply and demand?
Between 1940 and 2017, the US population has more than doubled.
In 1940, 3.8% of women and 5.5% of men in the USA completed 4 years of college.
In 2017, the corresponding numbers were 34.6% and 33.7%.
(https://www.statista.com/statistics/184272/educational-attainment-of-college-diploma-or-higher-by-gender/)
Meanwhile, how many new 4y colleges have been founded in the past 80 years (or even since ~1900)?