I think overall size of a population and population density have to be the contributing factors. More people means more people have traveled there from other places to spread the virus. More density means a higher R value.

The death rate per capita in Sweden is not much higher than the US. At ~33x the population the US has ~25x the deaths. Given the US had only double the total cases on March 6 as Sweden that seems like a positive indicator for Sweden's approach. It's like saying it doesn't really matter what you do so you might as well stay open. The timeline of when you started taking things seriously is also a big factor of course. I think it's way too early to declare anything yet, but I think it's good there are different approaches being taken to learn from.