First, and the most vague, is the idea of competent children in Sweden. I read an article recently (in the Swedish newspaper DN[1]) that traced a line through the author Ellen Key -> Astrid Lindgren and Rune Andréasson -> Pippi Longstocking and Bamse (comic book bear), and the idea that just because you're young doesn't mean that you're not competent. Both Pippi and Bamse talk directly to children (and the parents who read them to their children) and tell them about injustices and the idea that even a child can do things on an adult level.
This idea has been involved in the creation of preschools throughout the whole 20th century in Sweden. The newspaper article says that's why it's not surprising that Greta Thunberg is from Sweden, because you can't discount what someone does or says just because they're a child.
Also part of the preschool movement in Sweden was the idea that play is important for children. The DN article quotes the sisters Moberg who started the first Swedish preschool in 1904 as saying: "The child that plays a lot and with intensity, will surely become a competent human."
So children are expected to play and be children, but when they do and say things, they have been taught that it carries equal weight as what an adult does. It's a powerful idea that when children play (and I include teenagers playing video games or programming here, because a lot of parents would view it as playing), they're not just wasting time, but actively engaging in activities that will be useful for them in later life.
My idea is that young people in Sweden, as in many places, in the 80s and early 90s had a lot of unstructured free time. You weren't expected to have a lot of extra-curricular activities in Sweden, it was school, than do what you want.
(As an aside, unstructured free time has shrunk and is shrinking rapidly for children in Sweden, as well as elsewhere in the world. We now have organized play dates, soccer practice and swimming lessons and piano lessons, and so on. We had that in the 80s too, but the intensity is way up. What used to be one or two driven kids per school class is now the norm, so as a parent you have to fight to keep your kid's schedule free.)
What a lot of Swedish kids had was home computers! Here's where the earlier idea about competent children comes in, because a lot of parents would buy this machine and expect the kids to figure them out. Contrasting the C64 and Amiga 500 with ROM based systems like Atari 2600 and NES (NES was also huge in Sweden), the home computers required you to get more involved in how things worked when starting or copying games. They also came with Basic programming books, and taught you how to do a bouncing ball on screen, change the color of the ball, and so on. Pretty much a very basic demoscene tutorial.
It's hard to find exact sales number, but some articles say that the Commodore 64 sold 100 000 units in Sweden, and another article says that the Amiga 500 sold a lot better than the C64, and that the Amiga sold 100 000 units. Another article said that in the US, the Amiga 500 only sold 50 000 units in the first year, which was a disappointment. Comparing the populations of the US and Sweden in 1985 directly, without taking into account GDP, poverty levels, and so on, the US should sell 30 times the number of units as Sweden, all else being equal.
So per capita, a lot more young people would have a C64 and later an Amiga 500 in Sweden, both being primary computers used by the demoscene. The graphic and sound capabilities of the C64 and Amiga 500 made the demos possible.
Games for both computers were distributed on cheap cassettes and floppy disks. Pirated, because hardly any kids could afford to buy games at the store. So people had hundreds of pirated games on cassette and floppy disk, and usually the team that cracked it would put a short animation in the beginning bragging about cracking the game. This was the start of the demoscene, and they obviously had a lot of viewers.
Everyone I knew either had a C64 or Amiga or knew someone first hand who did. It was easy for anyone interested to follow a friend home after school to play games. And we all saw the cracker team demos, and were impressed by the star fields, rotating text, and chiptune music, so it's easy to see how someone could have become interested in doing that themselves, and started their own demo group.
I think that your final side note on the difference between the UK and Scandinavia is correct as well. I don't remember at all being worried about law enforcement getting involved with C64 and Amiga piracy in Sweden. This may have been because I was a kid and didn't read the news, but I think that the distance from the game producers, as you say, was involved. I don't even know if most of the teams who cracked the games were based in Sweden, or Germany or the UK, but I know that a lot of kids saw the cracker group intros in Sweden and went on to do full-length demos.
Personally, I still vividly remember the awe I felt when an older cousin showed me the State of the Art demo[2] on Amiga on my grandmother's TV in mid 90s. A bit later I saw the first Doom and thought that it was totally photo realistic.
[1] https://www.dn.se/kultur-noje/ingen-greta-utan-pippi-och-bam...
[2] https://www.youtube.com/watch?v=89wq5EoXy-0 (edit: I looked up now that the demo group Spaceballs was from Norway)