Brian spends almost all of Hatchet deep in the woods of Canada without any human interaction. It really doesn't get more isolated than that. (Or does it?) Part of the scariness of his situation comes from the fact that he's totally on his own, with no one to lean on, no one whose advice he can ask, no one with whom he can share his fears. Unless you count the bear, of course. But then again, Brian seems pretty isolated even before the plane crash. He barely speaks to his mother on the ride to the airport, and he's so angry and upset about his family's breakup that he's shutting out just about everybody. So which takes more of a toll: physical isolation or emotional isolation?
Brian's isolation is ultimately a good thing—it's exactly what he needs to get past the trauma of the breakup of his family.
Brian's sense of isolation isn't really based on his actual, physical isolation; in fact, he's far more isolated (emotionally) at the beginning of the book than at the end.