The question of “why Christ” is neither obvious nor uninteresting. The lack of an answer that withstands the passage of time suggests that this information is not really a practical necessity. On the other hand, it is a good exercise in the importance of context, and particularly historical context, in theology and biblical studies. To the NT authors who wrote on this subject, the issue was usually framed against the requirements of their own age. Put bluntly, the law was given by God. So why Christ?
A gift in place of a gift: the Fourth Gospel
The author of the Fourth Gospel addresses the matter in Jn 1:16-18. After recounting that John testified of Jesus’ preeminence and pre-existence, the narrator goes on to describe an effect of the Christ event: “from [Christ’s] fullness we have all received, grace in place of grace.” The gift or grace (charis) now received is Christ, and it came in place of another gift, that is, the law.
The narrator continues with his insight into God’s double gifting. God did give the law, but through Moses. A better gift came through a superior source:
17 because while the law was given through Moses, grace and truth came through Jesus Christ.
Finally, the narrator adds the nature of this superiority. The Son, a title used only of Christ in the Fourth Gospel, is superior because he has not only seen God, but enjoys (present tense) an intimacy with God like no other:
18 No one has ever seen God. The only Son, God, who is at the Father’s side (lit. “bosom”), has revealed him.
This idea of Christ as the superior revealer of God is critical since this is the Gospel in which eternal life is defined as knowing the “only true God, and Jesus Christ, whom [he] sent” (Jn 17:3). The revelation of God, of course, ultimately took place on the cross.
See what I mean about context? Next time, the view from the Epistle to the Hebrews.