I have a few ideas as to why it may not have been as busy in the past.
First, getting there. Most people who went to
Disneyland in the long-ago past didn't travel too far. There weren't super cheap flights with easy access to DLR like you can find today, and most families just didn't travel like that.
Second, many families were still single income.
Third, family dynamics in the past were not the same as they are today. Families vacationed differently--it was much less common to stay in hotels, have expensive meals, etc. Going to Disneyland was, for many (most?) families, a one-day experience.