Because, he was the first to be born again from SIN to immortality. Don't forget that the sins of the whole world (not just Adam as the WT tries to teach) were placed upon Him. He was the scape goat we read about in the OT. Yahweh abandoned him while on the cross. Why? Because Yahweh has no fellowship with SIN.
"He hath made him to be sin for us, who knew no sin; that we might be made the righteousness of God in him. 2 Cor. 5: 21
Thus, a legal transaction occurred. Believers trade places with Jesus - His perfect, undying body in exchange for our sin-riddled decaying body.
When He died, His soul descended into the lower parts of the Earth. Many people don't believe this, but I believe Jesus went to Sheol and was there for a time so that he could identify fully with the living and the dead. The Psalmist wrote of this in Psalms 16:10 For thou wilt not leave my soul in hell; neither wilt thou suffer thine Holy One to see corruption.
He led the way for us. He was the perfect scape-goat. He said, I lay it down and I take it back up. He did it willingly and then resurrected himself just like he said he would. And the course of human history was forever changed.
And now, "because He lives, we shall live also", "and shall be like him" incorruptible, permanently connected to the Source of life with no decay. He was born again from a disconnected, diminishing & sin-burdened life to a fully connected vibrant eternal life. That's why he is called that. It was a first.
"By his sacrificial death Jesus Christ bought back or redeemed all the human family who are dying "in Adam." (The Watchtower, June 15, 1979, page 22)
Only true for those who accept (with no strings attached like works or a particular church) his personal substitution of death and punishment on our behalf.
Amazing that Jesus would do this for us!