Because all that which is created is subject to God and beneath God's law.
Pro_22:7 The rich ruleth over the poor, and the borrower is servant to the lender.
Act_17:25 Neither is worshipped with men's hands, as though he needed any thing, seeing he giveth to all life, and breath, and all things;
Their (all created beings) life is not their own.
The divine Son (aka 'Jesus') of God [that is to say, the Father, aka 'Ancient of Days'] was the only sacrifice of sufficient value to fully satisfy the claims of God's perfect law. The angels were sinless, but of less value than the law of God. They were amenable to law. They were messengers to do the will of Christ, and before him to bow. They were created beings, and probationers. Upon Christ no requirements were laid. He had power to lay down his life, and to take it again. No obligation was laid upon him to undertake the work of atonement. It was a voluntary sacrifice that he made. His life was of sufficient value to rescue man from his fallen condition.
Not one of the angels could have become surety for the human race: their life is God's; they could not surrender it. The angels all wear the yoke of obedience. They are the appointed messengers of Him who is the commander of all heaven. But Christ is equal with God, infinite and omnipotent. He could pay the ransom for man's freedom. He is the eternal, self-existing Son, on whom no yoke had come; and when God asked, "Whom shall I send?" he could reply, "Here am I; send me." He could pledge himself to become man's surety; for he could say that which the highest angel could not say,--I have power over my own life, "power to lay it down, and . . . power to take it again."
Notice of the Son (by typology) it is written:
Num_19:2 This is the ordinance of the law which the LORD hath commanded, saying, Speak unto the children of Israel, that they bring thee a red heifer without spot, wherein is no blemish, and upon which never came yoke:
The Son's place was eternally (John 1:1-3; 1 John 1:1-3; Hebrews 1:8-12) by the side of the Father, upon the throne of Deity and "equal with" the Father (Philippians 2:6) in the very "form of God" (Philippians 2:6), with eternal glory (John 17:5) before taking upon Himself the "form of a servant" (Philippians 2:7).
A created angelic (perfect) being cannot take on human form and die for anyone. The entire WTS theology is bankrupt at this point.