See my reply to @Noyi to a similar question. ?.

Because it's a Christian nation and many of us are underground and we need someone to talk to!! The prejudice towards us is terrible.

I think because there are more english speaking americans on the net in general. However I begin to think the religiosity in the USA is quite oppressive and people need a space free of it.
We have lots of heathens in Oz, so less of an issue.

That's a good question .. Many friends in Euro land are atheists or agnostic .. Maybe they are not aware of this site or they do need one, since they are around more people with their view point.