Sforno on Deuteronomy 25:6/ספורנו על דברים כ״ה:ו׳

1ולא ימחה שמו מישראל, in that the child born from this union will be considered by G’d as if the deceased had fulfilled the commandment to be fruitful. This is easily understood when we consider that this child is the product of a legal marriage entered into by the deceased husband of his mother. The fact that the deceased’s brother did not have to go through a marriage ceremony with the widow of his brother makes this clear to everyone. This explains why G’d would be angry at the brother who refuses to marry his sister-in-law, as he thereby deprived his deceased brother forever from fulfilling the mitzvah of being fruitful