Maintaining archives of material is actually a pretty complex and expensive process, especially when we're talking about physical analog media (that is sometimes literally a ticking time bomb, as is the case with old cellulose film stock). Also, such media is expensive to acquire, so reusing them wherever possible is useful to keep costs down.
Combine this with the fact that the value of such preservation is often not apparent. Reruns of old material don't provide a lot of value--especially in an era where there are very few available channels--and there's no practical means for making copies of them to even interested buyers, at least in any sort of economical way (see above about cost of media). It's not until around 1980 or so that the home video market is a viable way to make money on old productions, and it's around that time that the wiping of old broadcasts was stopped as a regular practice.
