The reason is that dark, apocalyptic settings tend to do well in times of uncertainty and turmoil. The youth in such times tend to eat up mainstream nihilism with a spoon.
Keep in mind also that Cormac McCarthy's The Road is coming out in movie form later this summer. It's a fabulous, gritty, dark post-apocalyptic setting. The book is spectacular in its starkness. The movie is going to be a huge hit and probably nominated for a ton of Oscars and stuch. This year as the year of the Ballywood movie. 2009 will be the year of the apocalypse. Watchmen is also part of this. I'd look for other movies along the same line to come out. There is also something of a revival of the pulp novel going on. Wizards will totally want to capitalize on these elements and Darksun is a perfect fit. It's post-apocalyptic mixed with Edgar Rice Burroughs and it's likely to be a huge hit.