no really it's a good question. Year after year, the Fidler draw, MD and qualys both, is far stronger than it should be. I doubt that it's all in "the wonderful way the tournament treats the players" like Larson says, or in the fact that one player (invariably one of the top seeds) wins a car.
It might have more to do with Fidlerstadt benefiting from being the sole tour level women's tournament held during the entire year in the rich western half of Germany, which basically worships tennis players as stars like nowhere else in the world. So the stadium is always full, it's prestigious to play there, and it's convenient to get to, a lot of the European players can probably drive there. And the scheduling helps as well, it's far away from the Grand Slams and the week before Zurich, which is another very popular tournie.