Deuteronomy 25:5 "The wife of the dead man shall not marry an outsider"

If a woman's husband passes away and they had no children it is a Mitzvah for the brother of the dead husband to marry her (see Positive Mitzvah 216).

Since it is his opportunity to continue the family name of his dead brother, no one else is permitted to take the widow as a wife.

If he does not wish to marry her and fulfills the proper ceremony of "Chalitzah" (see Positive Mitzvah 217), the woman is free to marry someone else.