Because all the first-born are mine (see Exodus 13:2, and below on verse 43). That the powers of heaven had a special claim upon the firstling of man or beast was probably one of the oldest religious ideas in the world, which it would be difficult to trace to any origin but in some primeval revelation.
It branched out into many superstitions, of which the cruel cultus of Moloch was the worst. Among the tribes which preserved the patriarchal faith, it retained more or less of its primitive meaning in the assignment of sacrificial duties to the eldest son.
According to the Targums, the "young men of the children of Israel" sent by Moses to offer sacrifices before the consecration of Aaron (Exodus 24:5) were first-born. Whatever ancient and latent claims, however, God may have had upon the firstborn of Israel, they are here superseded by a special and recent claim founded upon their miraculous preservation when the first-born of the Egyptians were slain.
All the firstborn in that day became "anathema," devoted to God, for evil or for good, for death or for life. He, to whom belongs the whole harvest of human souls, came and claimed his first-fruits from the fields of Egypt.
He took unto himself by death the first-born of the Egyptians; he left for himself in life the first-born of the Israelites. For the convenience, however, of the people, and for the better and more regular discharge of the ministry, he was content to take the single small tribe of Levi in lieu of the first-born of all.