As odd as it is to consider for some, the original inventors of the game, Gary and Dave and everybody were full adults as was Dr. Holmes of Blue Book fame (the first truly mass market D&D in my opinion) , who was a respected surgeon . they were in modern terms were middle aged men in their 30 with careers, families and all the markers of adult hood.
This maturity allowed them to have very light rules, and to just in the words of Old Geezer (at 17 the youngster of the time) To Make up Some *bleep* they thought was fun
They were grown up enough to handle it.
And because they were grown ups not teen or shudder young teens, the cognitive skills they needed were well developed and as such "rulings not rules' worked.
This approach won't work with some player with maturity issues (there are lots of these alas) with with some kids and with some teens but no one considered kids much till the early 80's (Blue Book started it but B/X really took off)
I am guessing TSR s assumed that it would be imaginative adults that would play.
That of course changed as the hobby grew and and in the fullness if time D&D changed evolving into its opposite "rules not rulings" in 4e.
Now 5e from what i can tell is rolling back a bit either because they figure Old Grogs will be the main buyers or they realized that modern kids can in fact figure this stuff out and don't need to be spoon fed. Either way i am happy to see the game get back to its roots as in this mans opinion, the old way was better.
Bah, get off my lawn ...
The Return of Planescape? - Curious. There is a brief note on this 'countdown' at Enworld.
15 hours ago