Why does Islam grant the right of custody to the wife, but revoke it if she marries another man, citing the Prophet's saying, "Go, for he is yours as long as you do not marry"?

The wisdom behind this is summarized by the fact that if the custodial mother marries another man, she will be under his obedience and guardianship, and dedicated to him, which might lead to neglecting the child and not giving him adequate attention, especially if the new husband does not desire the child.