Because the Incarnation is the solution to all the problems of the Old Testament, from separation from God to the Nephilim to reclaiming all the nations – it is the center of the entire story. Jesus in His earthly life was human as we should have been and in His Resurrection is what we were always supposed to be. He is the fulfillment of every covenant and their foundation, the first true human, the key to re-opening Eden and picking up where things left off when the Serpent/Nachash tipped things off the rails.
Everything else in the scriptures is secondary to “Repent and believe”, and that rests on Jesus. Everything in all the texts is about Him, as He showed the two disciples on the way to Emmaus; as Paul put it, He is all in all, all things are for Him.
The scriptures are stories about YHWH-Elohim and His desire for a family made of matter, and Jesus is the pinacle of that because as the Firstborn/Way-Opener He shaped it all and forged a path where we will get shaped again into His likeness.