Not a lot of early Doctor Who episodes survived. The BBC was famously cheap and was always reusing its master tapes to save money. Fortunately they also put some of the early episodes on film reels and shipped them around the country to be shown in various places. Many of the early episodes available to us now were reconstructed from these film reels that were found in attics and church basements.

And the only reason Monty Python's Flying Circus survived was because someone at the BBC tipped off Terry Gilliam that the master tapes were about to be reused, and Gilliam was able to buy them himself and keep them in his garage.