The Mitzva of יבום, marrying one’s childless brother’s wife, is mentioned in פרשת כי תצא. The Torah tells us that such a marriage is a way of keeping the deceased brother’s name alive.
The question that is raised is regarding the son born to the widow and her brother in-law. This child knows nothing about his mother’s first husband, nor does he feel any connection to him. His father is the one who brought him into the world.
Rabbeinu Bechaye answers this question by saying that the words of the Torah, יקום על שם המת, shall be established by the name of the dead, is בנפש ולא בגוף, by the spirit and not the body.
The act of marrying the childless widow, is much more a Mitzva, felt in the Heavens, more than here in the physical world.
Perhaps this explains why the מצות יבום is no longer performed. We are no longer on the spiritual of understanding, to truly be able to perform יבום.