The archival story isn't quite correct, at least not according to the "25 Glorious Years" commemorative book from 1988.
Apparently there was an archival system in place, complete with a ranking system lettered A through J (where A stood for "prestige television to be kept perpetually"). Pre-1970 Doctor Who (the Hartnell and Troughton eras) had a C rank -- "retain selected episodes for posterity".
In 1972, the BBC archives were declared a fire hazard, necessitating a purge. And while selected pre-1970 episodes were retained, there really wasn't any rhyme or reason to the selection.
As an aside, the Monty Python troupe didn't believe that their material would rate on the archival ranking system at all, and made arrangements to store their master reels offsite. We almost certainly would have lost the first series of the Flying Circus if they hadn't....