Why?

Do you mean did Ada start talking?? Yes, she did. At the end she is practicing her speech on the porch of her house with Baines. I understood Flora's father to be the guy who caught light. She was not married to him, however; of the reasons I believed her father married her off to a stranger.

They don't give a concise answer as to why she stopped speaking. Flora's father was Ada's teacher (probably her piano teacher), Ada tells Flora while they're on the tent on the beach at night that he "understood Ada's thoughts without speech", but "became frightened and stopped listening", They weren't married and were probably separated by their families. The lightning story that Flora tells is obviously a fake tale.