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.