One thing to keep in mind is that in the 60's and 70's, we didn't have all this internet connection and telephones in the pocket. There were areas of the country (I was in one of them) where many people didn't even have phones in their homes because they didn't need them.
Neighbors kept an eye out for each other but mainly we were on our own a whole lot more, and if somebody hadn't been seen for a few days, nobody got alarmed. Seriously. It wasn't lack of caring. Missing work without letting somebody know would be somewhat more worrisome, but on the other hand, people and even families would decide to pull up stakes and try their luck somewhere else on quite short notice. Job in another state, creditors at the door, lost a job and no money to pay the rent. (It's still like that in what for lack of a better term I'll call the lower classes. One of my nieces has lived at six addresses and held four different jobs since last January.)
And if somebody said, "Hey, I'm going up to Alaska to work on the pipeline," packed his family into a small trailer, and left, all people would have said is, "I hope he can make it over the passes before the snow sets in." And if he came back alone, saying his wife left him and took the kids with her back to her mother in the south, nobody would have been surprised. True story. I've run into his kids since so I know he didn't murder his family and leave them beside the road in Yellow Knife or something, but at the time the possibility would never have crossed our minds.
It was the Vietnam era, and gay liberation. Families really did rupture and never talked to each other again. I know a man who on his deathbed still refused to make peace with his lesbian daughter. People went on the road in search of adventure or just to see the world. You didn't expect to hear from them for a couple of years (no telephones in the pockets, remember?) and if some of them didn't come back, you didn't know whether they were in jail in Alabama, married and settled down in Arizona, or dead beside the road somewhere. (This song kind of covers it:
https://www.youtube.com/watch?v=aTbBvPoxUkk)
And if somebody didn't come back, and you did want to try to find them, most people wouldn't have a clue about where to start looking. It's a big country. There weren't computers everywhere. You couldn't just search online databases or Google your loved one's name. You probably couldn't even get the police to take a missing persons report. And many people don't consider it a police matter that they've lost track of a loved one. For a lot of people, all that could be done is to wonder and wait.
So yeah, it's sad that these people can't be connected with their families, and some of it is because people didn't notice or care. But a lot of it is just circumstances. I don't know how to go about doing this, but I truly think the thing we could do that would lead to the most matches is to have some sort of push to try to get more people to report loved ones they've lost track of and get their DNA into the database.