Miles’s wife and children left because he did not age.
When the Tuck family first drank from the spring that made them immortal, they had no idea that immortality would be the effect. It took them awhile to realize that they were not aging and could not be killed. The oldest Tuck boy, Miles, had married and had children when his wife realized he was not aging.
“I was married. I had two children. But, from the look of me, I was still twenty-two. My wife, she finally made up her mind I'd sold my soul to the Devil. She left me. She went away and she took the children with her" (Ch. 7).
This was very sad for Miles, of course. He had done nothing wrong, and he lost his family. Jesse never married, because he was too young when he drank from the spring. As a result of the spring water, Miles and Jesse could never have families. All of the Tucks keep separate from most other people so that no one will notice they are different. It is a lonely life.
"It was the same with our friends," said Mae. "They come to pull back from us. There was talk about witchcraft. Black magic. Well, you can't hardly blame them, but finally we had to leave the farm. We didn't know where to go. We started back the way we come, just wandering” (Ch. 7).
The Tucks only stay in one place for a decade or so, and then leave. Even while they live somewhere, they avoid spending too much time in town. They don’t want people to notice their inability to age. The little house by the lake that Winnie admires so much is just a temporary home. Mae Tuck tells Winnie they will need to be moving on from there, too.