Definitions of disability changed a lot over these decades.
You might expect more. The US has had several more recent wars. Medical technology keeps a lot of people alive that would have died in the WWII era, leading to higher percentages of disabled civilians and soldiers.
In fact you might expect there to be more people on disability in 1960, seeing as WWII ended only 15 years before that (PTSD, etc). I suppose that those people simply couldn't get onto disability back then though.