I will say one thing. When you start in dwarf fortress, usually you have a year's breathing time before a siege comes knocking on your door. While you can influence your chances of survival in H&H, a large slice of it can come down to luck if a bear knocks you out and steals your shoes or not straight next to your hearth fire. I imagine that this can and does drive away a player population.
Likewise, new players wishing to start out together under the current system have no in-game way of knowing what the trapdoor, the ladder, and what sherlock does. Chances are they'll have to walk for hours through level X wilderness to play together or roll a new character. This prevents people from forming that whole society of players banding together that this game touts and is a glaring example of terrible game design. Be honest now, if you started out fresh on a new game and were greeted with that introductory screen, you'd just start clicking buttons until stuff happened, right?