Probably figured for that one brief scene viewed on a 21-inch home screen, with the TV transmission quality of the 1960s, no one would really notice.
21 inch screen?
I'm not sure, but that sounds high. This video of a 1965 Zenith TV shows a screen that is smaller than the last 19inch TV I bought.
(Wait, I just found a 1965 commercial for Zenith touting the biggest screen they had ever made at 25 inches, so 21 was probably a normal big screen of the day.)