I think Kira changes so much throughout the novel because of two reasons.
First, she has to change and adapt in order to survive. The culture that Kira lives in does not take kindly to people that cannot contribute to the wellbeing of the village. Babies that are born crippled are taken out into a field and left to die. People who are injured and unable to work are left out to die. Kira knows this. She should have been killed as a baby, because her leg is crippled a bit. Her mother is quite influential in the village and was able to keep Kira alive, but after Kira's mom dies, that protection isn't there anymore. Kira must adapt and change in order to survive.
The other reason that I think Kira changes so much is because of what she learns throughout the novel. She learns from her mother. She learns from her friends. But her education is most drastically affected after her mother dies. At that point she becomes a ward of the state and must live in the Council Edifice. It's there that she learns specifics about the superstitions of the village people and how to overcome her own fears. As she gains more understanding of the world around her, she can't help but be changed.
Kira grows and changes by realizing that her society isn't strong and stable. It keeps the willing people and kills the few people who are strong enough to realize that they are being undermined. She has to grow up and mature because she has to take care of herself and manage to sew the robe.