Because the New Covenant differs in radical ways from the Abrahamic Covenant and the subsequent covenants that Yahweh made with the biological descendants of Abraham.  The clearest passage is Jeremiah’s foretelling of it in chapter 31:31-34 of his book (cf. Ezek 36:26ff). 

“Behold, days are coming,” declares Yahweh, “when I will make a new covenant with the house of Israel and with the house of Judah, not like the covenant which I made with their fathers in the day I took them by the hand to bring them out of the land of Egypt, my covenant which they broke, although I was a husband to them,” declares Yahweh.  “But this is the covenant which I will make with the house of Israel after those days,” declares Yahweh, “I will put my law within them and on their heart I will write it; and I will be their God, and they shall be my people.  They will not teach again, each man his neighbor and each man his brother, saying, ‘Know Yahweh,’ for they will all know me, from the least of them to the greatest of them,” declares Yahweh, “for I will forgive their iniquity, and their sin I will remember no more.”

Every member/beneficiary of the New Covenant knows Yahweh savingly and is therefore forgiven.  No exceptions.  Ergo, only those who evidence this saving knowledge of Yahweh should be considered New Covenant members.  Hence only believers are to receive the signs of New Covenant membership—baptism and communion.   

My wife and I rear our children according to the law of God and we command our children to obey it, all the while knowing that they cannot and will not want to obey it until they are born again, i.e., become members/beneficiaries of the New Covenant.  New Covenant membership, as Hal Brunson often said rightly, is based on grace, not race.  Pedigree, ethnicity, or one’s seed line is absolutely irrelevant with the inauguration of the New Covenant.  The true, ultimate seed of Abraham has always been Jesus (Gal 3:16).  Now, the true sons of Israel/Isaac are those who have faith and those alone; see Romans 2:24-25 and Galatians 3:29.  Being the offspring of a Christian parent is now absolutely irrelevant. 

For more on this vital topic, I commend to you Alan Conner’s Covenant Children Today: Physical or Spiritual? and Gentry and Wellum’s Kingdom Through Covenant: A Biblical-Theological Understanding of the Covenants.  The former book is hard to beat for its length.