Aidan Stanger wrote:

I've often thought it amusing that so much paperwork is still kept in
the
BBC archives about these shows, yet the shows themselves have been
lost/disposed of.

Yeah but from the perspective of the late 1960s/early 1970s the need for
paper trails was clear, the need to retain old programmes that were
almost
certainly never going to be screened again and which were no longer
sellable
was not.

Why were they no longer sellable?

The rights had expired is the main reason. Most stories had something like a
five or seven year period in which they could be sold overseas, with an
option for one period to be renewed. After that what use were they?

Domestically agreements with the acting unions plus the switch to colour
meant that the propsect of repeats of loads of old back and white television
was very unlikely.

Domestic video recorders existed (the earliest known Doctor Who off air
videoing dates back to early 1969) but they were extremely expensive and I
don't think anyone had foreseen the era of cheap machines and the mass
selling of old programmes.