According to Daniel J. Flynn of the American Conservative (May 16, 2012):
Actors' guilds, state television, and mindless bureaucracy combined to do what enemies like the Master, the Cybermen, and even the Daleks never could: exterminate the Doctor. Arriving in 1963, at the peak of automation fears, "Doctor Who" became a casualty of labor's Luddite preoccupation with machine replacements. Britain's actors' guild, the nation's last purely closed shop, prohibited more than one re-broadcast of any program, lest its members lose work to on-screen facsimiles of themselves. The musicians' union and other trade guilds imposed similar restrictions. So '70s-era employees of the state broadcasting behemoth, imagining neither a world without a closed shop nor one with the VCR, trashed, lost, and wiped all of the series's '60s-era masters. The Doctor, like so many of that decade’s characters, was there but can’t much remember the 1960s.
This shocking example of how a labor union, in a misguided attempt to destroy competition, instead forever destroyed episodes from a classic TV show, is also reported in Wikipedia:
The actors' union Equity had actively fought against the introduction of TV recording since it originally became a practical proposition in the 1950s. Prior to the development of workable television recording, if a broadcaster wished to repeat a programme (usually a one-off play), the actors would be re-hired for an additional fee to perform it again live. Equity's concern was that if broadcasters were able to record the original performances, they would be able to repeat them indefinitely, which would cut down on the levels of new production and threaten the livelihoods of its members. Although Equity could not prevent recording altogether, it was able to stipulate that recordings could only be repeated a set number of times within a specific timeframe, and the fees payable for further use beyond that were deliberately so high that broadcasters would consider it unjustifiable to spend so much money repeating an old programme rather than making a new one. Consequently, recordings whose repeat rights had expired were considered to be of no further economic use to the broadcasters.
Because of these misguided labor practices -- similar to how today's actors unions support Big Media's expansion of copyright -- many of these classic TV episodes remain unavailable for viewing, though a few "lost episodes" have been found.