in fact, unfortunately, almost every TV station did that in that time, some of them preserved their material on film, but unfortunately not all of their material

TV station?

Dr Who wasn't a daily, TV station talk show, and Studios tend to archive.

The BBC gets the footnote in history, here.

No it happened in my country too. Technology was pretty shit in the 60s, TV was a new medium for European public broadcasters and they had to budget.