The ECE tests are just as stringent as DOT or Snell, and in many cases better.
But we have a lot more lawyers than you do.

So if someone hits a tree at 90 mph, our lawyers will claim that the helmet should have protected the inebriated individual so therefore helmet manufacturer must be at fault. BMW could easily end up spending more in liability costs than the profit they'd likely make. So BMW and a few other manufacturers just don't bother getting their helmets certified by the US agencies. Sad, but true.
BTW, I never had any trouble with my ECE-certified System IV helmets over many years and a lot of miles, and I feel they were as well constructed and as safe as anything offered over here.