Her mother never had the throne to give up. The line of succession went to Elizabeth when her father (the king) died. A ruling king or queen does not step down, they are there for life. The last time it happened was when Edward(?) gave up the throne back in the 1930s to marry Wallis Simpson, an American divorcee. This was a terrible scandal, as giving up the throne is just not done. I think it has only happened one other time in all of British history. You rule until you die.