Critics have been asking this same question since East of Eden was published in 1952. While none can agree that Steinbeck chose wisely, there is at least some understanding of why he chose as he did. The story tells a double account of the Trask family and the Hamilton family. Some chapters are devoted to the one and some are devoted to the other family. The difficulty comes in that Steinbeck made seemingly little or no or a merely inadequate attempt to blend the story lines or to integrate the stories of the two families: they are and always remain separate and distinct entities. It is felt by critics and readers that there should be some way in which their stories are integrated.
An example of this stylistic intent of non-integration can be seen in the narratorial choices Steinbeck makes. For instance, in Chapter 1 we encounter a first-person narrator who speaks at great length about the streams and Gabilan Mountains and live oaks of California's Salinas Valley. He then turns the tables and speaks of the dry years with little to no rainfall when the "land dried up ... and great bare scabby places appeared in the valley. ... and the sage-brush was gray." Then in Chapter 21, we encounter a limited third-person narrator who talks about Kate and Miss Faye's will:
Kate was in no hurry. She thought to the end very quickly then put it out of her mind ... Three of the girls questioned kate about the will ....
In conclusion, it seems Steinbeck may have been attempting to make a statement about the isolation and disordered condition of life and that he chose the novel structure as a sort of pictorial representation of this isolation and disorder. Yet contemporary critics and the original reviewers and critics of the story in general couldn't do much better than to ask the same question you have asked.