In an interview, Gregory Maguire gave a really interesting explanation of how he crafted the story of Wicked. Speaking of The Life of Emily Dickinson, by Richard Sewell, Maguire explains:
"Because Emily Dickinson was so private, Sewall had to find a new way to talk about her. The arrangement of his material – exploring all the people around her as a way to see toward the space she must inevitably occupy, like positing the existence of an invisible moon due to the gravitational pull it appears to be exerting – was a revelation, and helped me figure out how to organize Wicked." (source)
It's true that we hear more about Elphaba than from her. We constantly get outside perspectives and limited, biased information about her, but we don't see a lot of her in-depth thoughts or feelings. Even in the very last section, where we gain more access to her inner thoughts, she's referred to almost constantly by the narrator as "the Witch."
This limited and detached narrative perspective and style fit with the book's themes of storytelling, perspective, reputation, and biography. Wicked is not Elphaba's story, told from her point of view. She may be the star, but she doesn't really own her story, which is a key point made throughout the novel.
This distancing narrative technique heightens Elphaba's mysteriousness and her estrangement from others. It ties into the theme that the Wicked Witch of the West is a fiction, a creation not of Elphaba's own making, but that of others.