Because Christ called it "the fruit of the vine" and it would be unfermented. To properly represent the blood of the sinless Son of God, there should be nothing connected to leaven. It is interesting to note that in connection with the Lord's Supper, the word "wine" does not appear anywhere. Rather it is "the cup".
And he took the cup, and gave thanks, and gave it to them, saying, Drink ye all of it; For this is my blood of the New Testament, which is shed for many for the remission of sins. But I say unto you, I will not drink henceforth of this fruit of the vine, until that day when I drink it new with you in my Father's kingdom. (Mt 26:27-29)