Q. Why did Jesus choose only men as his twelve apostles?
I think that the number twelve is the key to the answer. I believe that Jesus named twelve apostles to be the leaders of the movement of his followers because he wanted to show symbolically that this movement would constitute a new community of God’s people. And so just as there were twelve tribal patriarchs for ancient Israel, the new community would have twelve “patriarchs” of its own.
The Bible gives us several indications of this continuity between the tribal patriarchs and the apostles. Jesus told the apostles at the Last Supper that they would “eat and drink at my table in my kingdom and sit on thrones, judging the twelve tribes of Israel.” As John describes his vision of the new Jerusalem in the book of Revelation, he says, “On the gates were written the names of the twelve tribes of Israel,” and then he adds, “The wall of the city had twelve foundations, and on them were the names of the twelve apostles of the Lamb.” And also in the book of Revelation, when John lists the tribes, as I argue in this post, “The names are presented in such a way as to show that the community of Jesus’ followers is the continuation of the people of God flowing out of the community of ancient Israel.”
So the selection of twelve men is, simply stated, symbolic. One implication of this is that I do not believe that the selection of only men as apostles means that women cannot have leadership roles in the community of Jesus’ followers. For my thoughts about that in greater detail, see the series of posts that begins here: Does the Bible say that women can’t teach or have authority over men? (Part 1). Indeed, Jesus chose not just twelve men, but specifically twelve Jewish men. But we do not conclude from that that non-Jewish people cannot have leadership roles in the community of his followers.