Why does the first scene in act one does not include Othello?
The play is about judgment and misjudgment. In the first scene, Iago and Roderigo discuss their feelings toward Othello. Iago characterizes Othello as a pompous man "loving his own pride and purposes." We learn that he passed over Iago who has field experience for a promotion and instead chose Cassio, a much less experienced man, for his lieutenant. We understand Iago's indignation. We learn that Othello has eloped with Desdemona and that Iago, Roderigo, and Brabantio disprove of this marriage. This first scene creates a negative image of Othello. We are not sure whose side we are on at the beginning of the play.
In the next scene, we find out that the negative view of Othello presented earlier in the play is wrong. Othello is an honorable and dignified general. He is not prone to fight, and would rather handle disputes calmly. He is genuinely in love with Desdemona, and is not afraid of defending his love. He did not kidnap her, but won her love through honorable means.
In a wonderful twist, Shakespeare shows us how even the audience can misjudge the hero and the villain, which leads nicely into Othello's misjudgment of Desdemona's faithfulness later on in the play.
By leaving Othello out of the first scene, the audience is introduced to the real Iago, the very evil and conniving man who is not just angry at Othello for not being promoted, but also just out to get him because he can. If Othello were present, it might be slightly confusing as Iago is going to play the role of loyal subordinate, this way we get to see him at his evil best.
The second thing that is interesting/useful to leaving Othello out of the scene is the introduction of the generally racist nature of Venice and her ruling class. Iago is able to get Desdemona's father roaring mad by proposing that she is out cavorting with a black man, a moor.