Parable, Historical Fiction, Realism
Historical Fiction
Buck's novel is just coincidentally historical fiction. In most historical fiction, history plays a big role in the plot and in the characters' lives, but in The Good Earth, history is mostly background decoration. The story could happen anywhere, at any time.
Think about it. Would Wang Lung’s story be out of place in the farmlands of the USA? Or how about Venezuela? France? No matter where he lived, or what he was named, Wang Lung’s life would still be connected to the land. We’re also pretty sure there are lots of stubborn, ambitious, hardheaded guys all over the world.
Still, the novel does try to make you feel like you are in the Chinese countryside at the turn of the 20th century, just like most historical fiction tries to make you feel you are experiencing a specific historical epoch. It's a story set in a particular place and time featuring characters and themes that could exist in any place or time. Think about it as sort of like Downton Abbey but set in early 20th century China instead of prewar England.
Realism
Part of what made Buck's novel so famous was her use of realism. She writes about the real, everyday life of normal people in China. Until then, most literature in America about China was romanticized, unrealistic, or about extraordinary people.
Buck drew from her experience of growing up in China to provide realistic details about the country that most Western authors couldn't know about. She also created a story about a farmer who is so universal that he could exist anywhere, not just in China. Audiences were drawn to reading The Good Earth because they could feel like they were getting "the real deal."
Parable
The moral of this story is pretty obvious. It's not complicated, and it's not some fancy literary fiction theorizing. Nope, we can boil it down to three words: remember your roots. If you want to get fancy, you can add a second moral: loyalty trumps beauty. Still, it's only six words in total.
Even though The Good Earth is a very long book, it has a lot in common with the parable, a short story with a clear moral. Buck, of course, takes makes the parable a lot more complicated: for example, even though her message is simple, she elevates the genre of parable by creating characters who are complex and who have complicated relationships. Instead of being stock characters, Wang Lung and O-lan actually surprise us sometimes.
Buck uses style of the parable because of its similarity to traditional Chinese storytelling. In the West, most people are exposed to parables through Bible stories like "The Good Samaritan" and "The Prodigal Son," but in pre-revolutionary China, parables were used to educate and teach people about morals. Especially as a kid growing up in China, Buck probably heard lots of Chinese parables before bed time, so using this genre helps her evoke the feeling of being in China.