Why Prince William and not Prince Charles?

The line of succession always goes to the Monarch's eldest child. If the monarch has no children, it then goes to the eldest sibling. If there are no siblings, it goes to the Eldest Aunt or Uncle and so on...

Prince Charles is Queen Elizabeth's Eldest child, so he is next in line.

Prince William is Prince Charles' Eldest Child so he will follow Charles.

Prince George is Prince William's Eldest Child, so he will follow William.