That was only the first time, and only after the virus was everywhere. SK reacted much more rapidly when it initially came to them.
Furthermore, Schengen. SK are for all intents and purposes an island, and everything and everyone must come by either boat or plane, which helps with control.
It depends on the disease, covid is immensely correlated with diabetes / metabolic syndrome, which is rampant in many western countries with America at the top.