First question on my mind would be, where are you getting your information? I can't find IVAO stats anywhere on their website and the last time their ServInfo updated was 1/19.

In any case, yes, IVAO might beat us in Europe...but what about the rest of the world? From my limited knowledge of IVAO, the USA Division is far stronger in terms of controller+pilot numbers.

I suppose the best answer I could give you about IVAO's success in Europe is that it's busy...traffic is self-perpetuating if you know what I mean. They get more traffic because there's more traffic to fly around. This is similar to why FNO is so popular and why whenever an airport gets a blizzard they get slammed with traffic.  Anything for a higher risk of slamming virtual metal into other virtual metal, eh? Or in Denver's case, virtual metal into virtual ground