Why was Joseph the father of Jesus?

It's not known what happened to Joseph. He is present at the Temple when Jesus astounded the scholars in Luke 2:41-52, but since Mary is described at the crucifixion and the post-ascension scene in Acts 1 by herself, it's thought he had died somewhere between. It's unlikely he would fall away after seeing and experience similar things that Mary had. Jude and James talked about in 1 Cor. 15 and Galatians 2 as being the "brothers of the Lord," are thought to be His half-brothers, by a second husband after Joseph. That would put Joseph's death somewhere between A.D. 11-29.