Answer: Because he is introducing the readers to the story and doesn't want to give too much information.
Explanation: In Chapter 1 of "The War of the Worlds" by H. G. Wells, the narrator calls the Martian spacecraft the "Thing", because he is introducing the readers into the story and building suspense. He is in the exposition part of the plot. It happened when Mars was closer to the Earth. There were some flashes of light coming from the Red Planet's surface. Then this "Thing... an unerring missile" came to the Earth and caused a lot of suffering and struggle.