This is going to sound silly, but up until 1978, the BBC didn’t put a high priority on keeping archives of their old TV series. As a result, a _lot_ of TV shows from the dawn of British television through the mid-1970s have been erased and gone missing. Roughly speaking, about 40% of the first six seasons of Doctor Who have gone missing. It’s so bad, only 6 of the 21 storylines with second Doctor Patrick Troughton have all their episodes in possession of the BBC.