Why exactly are all new HMHVV II forms first encountered in North America? 
Because when I wrote the thing originally, back for SR3 about, oh, seven years or so ago (I'd have to go check my archives for a more exact figure), I wasn't getting much in the way of cooperation from European freelancers, or the European branch of the company that held the license (that would be FanPro). Mostly FanPro. Being of a grudge-holding bent at the time, I said to myself, "Self...fuck it." And they were all found in the Western Hemisphere from that point.
I
almost had harvesters appearing in Sao Paolo, but changed my mind since I wanted something to be from Texas. Not enough was known when I first wrote the thing about Asamondo so the gnawer, which I wanted to go alongside the ghoul, was first found in Chicago. Can't recall offhand why grendels turned up in Pittsburgh first. I think I was watching a football game at the time, and the Steelers were playing.
If it makes you feel better, you can just say that it's because I'm an America-centric jerk. I'm not apologizing for it, however.