Because one would represent Yeshua's blood purifying the heavenly things and the other would represent Yeshua's removal of our sins.
To teach us that man's Atonement is totally YHWH's doing.
The fact that both goats had to be unblemished before lots were drawn shows neither one could represent Satan. Neither can Satan be an atonement for sin. There are many verses showing that was Yeshua's honor.
Leviticus 16:22; "And the goat shall
bear upon him all their iniquities unto a land not inhabited: and he shall let go the goat in the wilderness."
Now consider the following;
Isaiah 53:6c - "And YHWH hath laid on him (Yeshua) the iniquity of us all."
The sins of Israel were laid on Yeshua. They were placed upon him and him alone.
Yeshua is the one who bore our sins. He carried them away upon his own head which is what the Hebrew 'cabal' (bare) means.
Isaiah 53:4a - "Surely he (Yeshua) hath borne our griefs, and carried our sorrows:"
Yeshua is the sin-bearer, not Satan.
Hebrews 9:28a - "So Messiah was once offered to bear the sins of many;"
1 Peter 2:24a - "Who his own self bare our sins in his own body on the tree."
There is no other sin-bearer.
The act of laying on of hands (vs.21; also;
Leviticus 1:4;
3:2;
4:4,
15,
29,
33) symbolizes the transference of sins from the guilty party (the children of Israel) to the innocent (azazel). The innocent then becomes the sin-bearer. Yeshua undeniably fulfills the type (
Isaiah 53:4,
6,
11,
12). Satan, however, cannot fulfill the type because he is not innocent. It will not satisfy the justice of YHWH to transfer the sins of the guilty to another guilty party. Since lots were used to decide which goat was "for YHWH," it meant that both goats had to be unblemished. The antitype of an unblemished goat was the sinlessness of Messiah. Can it be said that Satan is sinless and was to be represented by an unblemished goat?