How did Esther become queen?

By pleasing the king more than all the other fair young virgins in his kingdom

Let there be fair young virgins sought for the king. ...

And let the maiden which pleaseth the king be queen instead of Vashti. And the thing pleased the king; and he did so. ...

And the king loved Esther above all the women, and she obtained grace and favour in his sight more than all the virgins; so that he set the royal crown upon her head, and made her queen instead of Vashti. Esther 2:2-17