Student loan debt now is also far more common and larger as a percentage of expected income than it was back in the 70s. The University of California didn't start charging residents tuition at all until the mid-1970s and didn't really ramp up until Governor Reagan slashed the budget:

http://www.dailycal.org/2014/12/22/history-uc-tuition-since-...

If random internet sites are to be believed, the $630 combined tuition plus fees in 1976 is roughly $2k today. An awful lot of students could manage that without aid.