Well, because it predicts real science, and uses real science. Such as centrifugal force, real-time space travel, effects of gravity (the tall child on the moon base), use of Velcro, space food (another gravity issue), and AI computers, taking into account the history of computers (such as HAL singing "Daisy Bell" in an allusion to the IBM 7094). It not only uses real science to build a future science, it also is an excellently written novel with a compelling plot. It has insight not only into science, but into human beings in a space age. The human element is the strongest element of the novel, which is what I think makes the book so compelling.