Because Sweden, like Japan, follows the government guidelines on masks + social distancing and also already is an introverted culture. They also tend to care about their fellow citizens instead of just themselves. And many live in single family households which helps reduce the second branch of spread. Corona also spreads through super spreaders so if your culture is introverted you will have a lot less of them. Think of like Sex and the City type people who basically go out every night and are constantly talking/singing/yelling when they are out at bars/events.

So what works in Sweden/Japan won't work in some cultures like Italy who are very extroverted and greet people through close contact. Or the US/UK who don't trust their governments, only care about themselves and their family, and also physical in greetings.