Partly because the disabled who needed it just didn’t get out much back then.
If you have to get down & back up a curb every single block, and then most stores have a step or two into the store, and narrow, unpowered doors, and most restaurants & theaters can’t accommodate you, you soon find going out more trouble than it’s worth.
It took that long for Bob Dole to gain the necessary stature and position in the Senate to pass the ADA. Prior to his support, the cost and intrusion into daily life prevented passage. When Sen. Dole was Majority leader, he got it passed.
The statistics are from the bureau of labor statistics and my understanding is that they conduct a survey asking if a person is employed not whether they are actively looking.
The predecessor to the ADA, as touched on by RivkahChaya, was the Rehabilitation Act of 1973, which is early enough for the OP’s sentiment. 
But the Rehab Act applied only to the federal government, federal-gov’t contractors, and entities receiving federal financial assistance, while the ADA applies more broadly.
Things come in their own time, right? I know it sounds like I am being a smartass, but things are not passed until they are passed. We might as well ask why it took until 2015 to legalize gay marriage or until 2063 to ban animal slaughter. Or why did it take until the 60s for civil rights laws instead of the 30s?
Is there something deeper I am missing?
I see that the OP it says “Most landmark legislation involving social justice was passed in the 1960’s and 70’s even. Medicare, Medicaid, the fair housing act. Why did it take so long to Pass the ADA?” I suppose OP considered it unnecessary to mention the Voting Rights Act and Civil Rights Act but let’s put them there as well as being more to the point. (After all you could end racial segregation, extend voting rights, and order that there be equal housing and employment opportunity without providing health care coverages or food stamps.)
The thing is, there is no reason why ALL “social justice legislation” must happen at the same time. Not even in the 60s, which were not the all-encompassing wave of revolutionary enlightenment some want to imagine. People can some times come under the impression that there HAD to be some sort of necessary total intersectionality among all aspects of civil rights and social justice law and cultural opinion in both time and space.
As mentioned earlier, *incremental *legislation about disabled rights *had *been passing since the 70s. That is as much a legit way of advancing justice. The one single big, broad high-profile piece of national legislation that really put everyone on the same page took until the late 80s to come into its own, largely backlash-free. Not that bad in the larger scheme of things.