That's because Dr. Anders Tegnell, their chief medical guy, basically said at the start of the pandemic: "The vulnerable and the elderly can take precautions if they wish and feel it's appropriate to do so, the rest of us will get on with our lives". The virus burned through their population and approximately 13% of their population were known cases (and probably about 2x or 3x probably were exposed to it but didn't bother getting tested).