Because the Lord uses here a rhetorico-ironical means of disputation and education: in this context, the Jews are affirming that they are not slaves, in virtue of the fact, that they are "children/descendants of Abraham" (8:33). Since the Lord confronts exactly this claim of Jews, He tells them that Abraham, their father, did not oppose Him, but on the contrary, rejoiced in Him (John 8:56). If so, then the sentence acquires a new rhetorico-ironic dimension, implying that they cannot legitimately claim the sonship of Abraham, given the fact that their behavior and disposition is totally, radically, at odds with those of Abraham. A hidden premise for reaching this conclusion is that a real, legitimate son and heir will also inherit the behavior and disposition of the father.
Therefore we can read the Lord's sentence like that: "You claim to be free in virtue of believing to be descendants of Abraham, however, you are not behaving freely, like Abraham, the essence of whose freedom was expressed in the fact that He rejoiced in Me, for only a man free from sinfulness or eager to overcome sinfulness can be considered as free or in the process of getting freedom; you, on the contrary, deny the essence of Abraham's freedom, which is rejoicing in Me, in quite a radical way, desiring to kill Me; this cannot be the sign of freedom, but rather, a sign of sinfulness and slavery, for everybody who sins is a slave of sin (John 8:34); therefore, think for yourself, are you really children of Abraham? Is the genetical/blood affinity enough to make such a claim when spiritually you deny altogether Abraham's morality and disposition?" Since the question is addressed only to those who claim sonship of Abraham and simultaneously betray morality and disposition of Abraham, Christ cannot include Himself in this class, and thus He says "your father Abraham" and not "our father Abraham".