Bill, you’re exactly right. And something you’ve probably thought of but didn’t mention that applies particularly in this case is fear. (There has been a lot of speculation about conspiracy and mysterious deaths surrounding the assassination.) Many witnesses were never contacted. And some were deeply traumatized by the events.
I wouldn’t even try to contact anyone who has indicated a preference for privacy. The same for anyone who I think was traumatized.
Edit: Howard Brennan is a hero for his decision to get involved. His description of Oswald was crucial. And there is a reasonable chance that he could have recognized Oswald as he walked out of the TSBD and pointed him out if he hadn’t been distracted by unwanted reporters.
Hugh Aynesworth, the Dallas reporter who was one of if not the first on the scene at the TSBD after the shooting, gives this account of him trying to interview people (this is from his book "Eyewitness to History"):
"Some witnesses hurriedly shared a few comments with me. Some feared being quoted by name. One woman said she worked in the depository building, "and I'll be damned if I am going to tell you what I believe..."
And this: "I saw [Howard] Brennan talking to two officers and tried to poke my nose into the conversation. "I saw him up there in the window," I heard him say as he pointed toward Oswald's sniper's nest. "No doubt he was the one. He wasn't even in much of a hurry."
One cop asked if Brennan could describe the shooter. "Of course", he answered, "I saw him real good."
Then Brennan noticed me and moved away, asking the officers as he did so to keep me and the other reporters away from him--a request they were glad to fulfil. Brennan, I later learned, feared talking to the press else he endganger himself or his family.