The house is the main character in Bradbury's "There Will Come Soft Rains" because its human occupants and the rest of the humans in the area have been killed.
The house, a technological marvel that takes care of all of the family's needs, is designed to seem like a person. For example, it sings to the family as a human would: "Rain, rain, go away; umbrellas, raincoats for today."
However, the narrator of the story also personifies the house, for example, by writing:
Until this day, how well the house had kept its peace. ... it had shut up its windows and drawn shades in an old-maidenly preoccupation with self-protection which bordered on a mechanical paranoia.
It quivered at each sound, the house did. If a sparrow brushed a window, the shade snapped up. The bird, startled, flew off! No, not even a bird must touch the house!
The house in the quote above is likened to a human being keeping its peace, which means not stirring up trouble. The narrator compares it to an older, unmarried woman concerned to protect itself and continues the personification with the idea that the house "quivered" at noises, as a frightened person might.
At night, "the house began to die." This is another personification, as a house can shut down or malfunction, but it cannot die liking a living person. However, we have become so used to regarding it as a person that this wording seems natural.
As the house burns, it continues to be treated as a human being:
"Fire!" screamed a voice. The house lights flashed, water pumps shot water from the ceilings. But the solvent spread on the linoleum, licking, eating, under the kitchen door, while the voices took it up in chorus: "Fire, fire, fire!"
The house tried to save itself.
Above, the tapes that run the house are referred to as a "voice" or "voices." The sentence saying that the "house tried to save itself" treats the house as if it has human will or agency.
As the fire tears through the house, it is again personified, this time in vivid terms as a human body:
Its bared skeleton cringing from the heat, its wire, its nerves revealed as if a surgeon had torn the skin off to let the red veins and capillaries quiver in the scalded air.