Why didn't more people graduate with a college degree when it was "affordable"? Adjusting for inflation, (without too much controversy, I expect), it is evident that college was more affordable in the past.  See for example, this chart of Tuition and Fees and Room and Board over Time, 1975-76 to 2015-16.

And yet, in the United States in 1980, roughly only 15% of the population had a college degree. It seems to me to be a very economical choice to attend.  Are there reasons for the low graduation rate?
There are many factors!


In the past there was much less demand for highly educated people, as there were many more jobs that were mostly manual labour. Technology is largely to blame for this, as computers and machines take over what we had to do with our heads and hands.
People were more likely to work in one job for their whole life, so there was less incentive to get a more general education.
Apprenticeships were more common, so you'd learn on the job rather than study at university.
It was easier in the past to "work your way up the company" – you'd start as a front-line grunt, and have a good chance of becoming a manager later on.
Children with poorly educated parents are much less likely to go to university. Starting off 100 years ago when almost everyone was poorly educated, it's actually taken remarkably few generations to get where we are now.