There are many reasons babies and children get adopted. Sometimes when babies are born, their birth parents are too young to take good care of them and want them to have parents who can give them a better life. Sometimes the birth parents are sick or they even died and the child is an orphan. Or parents are extremely poor and can't by their baby enough food. Other times the birth parents don't have the support of their family to raise a baby.
It's important to know that adoption is a decision made by the grown-ups. Being adopted has nothing to do with anything the baby or child did or didn't do. Children who are adopted didn't do anything bad or wrong. Parents who adopt children love their children very much. Often they are called "Forever Parents" by children who get adopted.