Well spotted! This is an interesting fact about this excellent book concerning migration and the expansion of the border of the West in America, and is an issue that has troubled critics. The structure of the book and the way that Antonia herself is apparently forgotten about for an entire section does lead some to question who the actual protagonist of the novel is supposed to be. Lena Lingard, in addition, is said by some to be a much more interesting and fascinating character compared to the somewhat bland Antonia, and so some argue that Lena Lingard actually threatens to "steal teh show."
In response to this issue, David Daiches, a famous critic of Cather and her works, actually believes that this is a fault of Cather in this brilliant novel, and that she loses focus of her central theme, which he said to be the "development and self-discovery of the heroine." In addition, others have pointed toards the mode of narration to explain the curioius lack of focus. Having a male narrator who is infatuated and fascinated with the central character but who remains profoundly detached from so much of the action results in a lack of focus and a kind of moral emptiness at the centre of the novel, some argue.
You might want to think about this issue based on your own reading of the novel. Who is the true heroine? Which out of the two female characters it the more appealing and why? Why do you think Cather left Antonia for a whole section to follow a different character?