I imagine the reasoning went something like:
We have loads of old black and white stuff on 2" video tape. In this age of colour TV no one would ever want to see those again. In addition they take up miles of expensive storage. If we record over them we save on our tape budget and free up space too....
If the situation with Doctor Who is appalling, then early Top Of The Pops is even worse. Almost nothing exists of early TOTP - no Beatles, 1 performance by The Stones, a rehearsal take of Sandie Shaw and so on, because no-one high up in the Beeb realised just how important pop music would prove to be.
Ironically I think every TV News bulletin has been kept, because news is "important".
Luckily the germans preserved Beat Club and Beat Beat Beat so we still have some pop.