Probably a pragmatic choice. High school kids schedules are absolutely packed with focused study for exams and then more after school homework and cram schools. If you told parents and students they were going to have less study time to get ready for university exams they'd probably be a lot of pushback.

Elementary school students have much more flexible time, and university seems like it's a long way away. Taking a chance on computing in school won't seem nearly as scary to parents.

Because it's useful to know. If you wait until college to learn it then you miss out on years of use. Having kids get experience with it early on opens their options too. It makes it a less daunting career option. I wish I had exposure earlier on. If I did then I definitely would have gone for CS rather than mechanical engineering.

Keep in mind that elementary school kids in Japan also attend evening "cram" schools, where they drill on the subjects on the curriculum. I wouldn't be surprised if the real learning happens at Kumon, etc. By the time they're in high school, it seems like it's all about passing tests and getting into college, but I never went to Japanese school, so my perspective could be totally wrong.

It would be idea to start in secondary/high school as in educations in Singapore does.

what do you mean top down?