Music is an enormous factor in movies, I wonder why nobody mentions it. For example, the Lord of the Rings soundtrack is spectacular and adds greatly to the pleasure in watching it. In contrast, the soundtrack to The Hobbit sounds completely generic and boring, and the result is unwatchable.
Another example is Star Wars. The first two movies had amazingly good soundtracks. The later sequels had boring music, and whaddya know, the sequels were boring.
The Hackers soundtrack made more money than the actual movie. There was a sequel to the soundtrack but not the movie and unless you are involved in development, almost nobody knows the movie but you still hear songs from the soundtracks at nightclubs today.
I'm having trouble coming up with an example, but my dad told me that "Warsaw Concerto" was composed for the movie "Dangerous Moonlight". The movie was bad, but was popular because people really liked "Warsaw Concerto".
I could probably be proved wrong( I don’t think it’s entirely causal but more “they won’t pay for good music if they won’t pay for a good script” kind of thing.
That said, I would absolutely buy (and wear) a Xanadu jacket if I saw one for a reasonable price.
It just wasn't an issue that was seriously considered by a lot of studios(?) at the time and it's not like back catalog TV shows are usually these big money-makers that warrant a lot of time and cost to get in order.
And think of how few people will watch the show solely because it features copyright music.
It should be the other way around, i.e. Stranger Things should send the record company a bill for the resurgence of "Running up that hill".