Where do we begin? This whole book is an allegory for the first few chapters of the Book of Genesis. Okay, so we should probably begin there, but first let's talk about why Steinbeck would go through all the trouble of writing a six-hundred-page novel about a story that Genesis tells in four chapters.
Midway through the novel Lee tells us that the story of Cain and Abel is important because "it is everybody's story" (22.4.64). In other words, the story about Cain and Abel isn't just limited to Cain and Abel: it's actually a kind of parable about how we as humans generally react when we are jealous. So Steinbeck is giving us a modernized version of that same story, and because the feeling of jealousy is so universal, it totally works. Readers today might have a hard time relating to Cain, but they sure as heck can relate to Cal. And being able to relate to a universal figure like Cal makes us think about what it means to be a human and to feel those kinds of emotions.
But why update this tale as old as time at all? Because it's not like a whole lot has changed in thousands of years in terms of how we as humans feel and behave. It's crazy to think that when the Bible was written, siblings were jealous of one another just as they are today, that parents had their favorites, and that there was family drama. That's because this novel has a big stake in the past, and you'll notice that throughout East of Eden there are a lot of discussions about how our notion of the past affects us. (We talk about some of them in our "Themes" section.)