What I find interesting is that the vast majority of users are in the US. I wuold have expected a 2-fold or even 5-fold increase over the next countries (in EU), but if I look for example at Germany, UK or France, there's a 10x to 50x discrepancy.

Seeing as those countries are usually very much into emulation of older systems, were flash games simply not that poular outside of the US?

Personally, for example, I had some flash games that I played, but it was definitely a niche at least among my friends/family at that time, even at school or university.