I've been here before...I used to be a fan of the 1970s TV series "Colditz" but for some reason it was never ever repeated, except for one single episode (the one where one of the prisoner feigns madness and eventually becomes deranged).
On the radio, I used to enjoy repeats of the 1950s series "Hancock's Half Hour" but they only ever repeated shows from the last three series. (Of course, I later discovered that the BBC no longer had them. They have since been retrieved from various sources.)
The tricky problem of repeat rights is sometimes behind this problem. Maybe other ABs who know more about this than I do can elucidate.