In Ray Bradbury's "All Summer in a Day," the only evidence as to why the children are attending school and living on Venus is this simple explanation:
"And this was the way life was forever on the planet Venus, and this was the schoolroom of the children of the rocket men and women who had come to a raining world to set up civilization and live out their lives."
It seems from the above passage that there were rocket pioneers who settled the world and set up a civilization. Why did other pioneers and pilgrims leave their homes in search for other places to live? There are many reasons from God, Gold, and Glory like it says in the history books (New World Era) or for any number of reasons; but that passage in the second full paragraph after the opening dialogue of the story is all the explanation Bradbury gives.