What's been recovered has been occasionally the code from contracted PC ports.
I cannot claim to know why they did this, but my educated/experienced guess as a long-time game developer is that _someone_ thought it would be best for the teams not to rely upon existing projects when creating new ones.
Imagine if painters always started from their previous painting rather than a blank canvas; or if a home developer always started with prefabs of their previous home. You'd get American suburbs and Ikea prints.
And video releases weren't yet a thing even if the rights could have been secured, so the apparent value of these archives was minimal.
"Talent unions were highly suspicious of the threat to new work if programmes were repeated; indeed, before 1955 Equity insisted that any telerecording made (of a repeat performance) could only "be viewed privately" on BBC premises and not transmitted"
I'm trying to work out if there's a misunderstanding here, as this only applies to repeat performances, not presumably the original performance. So I wonder if recording at all at that time was not standard practise. I tried to work out in what circumstances telerecording was actually used for at that time, havent found much, but found this interesting white paper on recording the Queens coronation.
So telerecording was just a weird hack, and for a repeat performance you would expect to bring the actors into the studio again.
Which, if true, makes me earnestly sick to my stomach.