the short answer is - internet is available everywhere.
I think, the practical reason is that, when any english speaking person around the world has social anxiety and wants to find forums for social anxiety, he/she would just type "social anxiety forum" in google.
Once you do that this website is the one that would pop up in the list.
So people just join up.
If there is a forum in UK for this kind of thing, then you would find people from all over the world signing up there.
The only distinction is that you find only english speaking people here.