Three-body is soft sci-fi, why do people say it's hard sci-fi?

The Three-Body Problem was a Chinese science fiction novel by Liu Cixin. Although it has some sci-fi elements, it is more of an exploration of sociology, history and philosophy, so some people think that it is not pure science fiction. In the Three-Body, humanity faced the threat of an alien civilization. This threat came from a cosmic structure called the Three-Body. There was a mysterious connection between the three components of the universe's structure, which allowed the Three-Body Civilizations to continuously develop, expand, and attack other civilizations. Although there were some sci-fi elements such as extraterrestrial life and time travel, it also involved more realistic social issues and philosophical thinking such as the fate of mankind, the development of civilization, and the process of history. Therefore, some people thought that the Three-Body Problem had some sci-fi elements and some real-life revelations, so it was called hard sci-fi.