Well, duh. The book's named after her. Ella is the protagonist of the novel for a boatload of reasons: she's the main point-of-view character so we come to know her and care about her concerns. She's good at heart so we can sympathize with her. She's downtrodden so we're rooting for her to succeed in throwing off the yoke of her curse and her nasty stepfamily (who doesn't love an underdog?).
Finally, she interacts with a host of other characters and engages with the world around her, so she's the linchpin connecting a bunch of subplots: her father's economic scheming, her mother's connection with fairies, her stepfamily's greed, Char's skirmish with the ogres and general good-heartedness, and Lucinda's reformation. Just to name a few.