I think 2HBH is prevalent because it helps the kids to win more in the 8s, 10s, 12s, and even 14s.
My son just turned 9, and has been using a 1HBH since he was 5. I think people try to have the kids hit topspin 1HBH and give up. I started him off with the old fashioned underspin drive like Rosewall and many of the other players did in the 70s. I make him practice the topspin 1HBH often, but for matches he resorts to the Rosewall drive. It is natural for him since we volley a lot, and the Rosewall underspin drive is pretty much an extension of his volleys.