Well, what did you expect of the world of the Gods that govern the dead? Historical speaking, the world of the dead and their gods live in a somber place. The author itself seems to just picture it like that without a real "motif" of the why but simply because it should be do like that:
Obata said that he used "no real design motif" for the Shinigami Realm, and that he never settled on any concrete appearance; Obata described the Realm as changing appearance in each instance in Death Note, with it sometimes appearing as a dry field and sometimes appearing as a room "full of cookies." Obata says that he likes to think of the Shinigami Realm as "an abandoned building with chunks of chocolate sitting around." Obata said that he thought of the realm as being "inside something" and having a "claustrophobic feeling." Obata added that he would like to develop the Shinigami Realm further if it was used as the setting for a story.
I don't have the "Takeshi Obata Production Note: Characters." Death Note 13: How to Read. book to verify the Wikipedia source but if you buy it it should tell the same.