My guess is because it's not popular enough yet so there isn't that "herd" effect. When I first made a facebook I was among the first in my country and everyone was like "pfffff facebook, what's that shit" since everybody was using high-5, myspace and some other sites the names of which I don't remember anymore!
Functionality of the site is another reason, hopefully this will change with communities :)
Your right about this for sure. Once it's the big thing and everyone is doing it, we will definitely see much more adoption.