It's the predominant grappling art in their country; not a huge mystery. Hence, the name "BRAZILIAN Ju Jitsu".
Edit: I honestly don't remember a time when Americans were known to be the best sub artists. I mean, sure, Mir, Hughes, etc have great sub games, but for every one of them, there were likely 5 Brazilians with comparable submission grappling in the sport, at any given time.