Because at the time of voting for the nominations, ARGO wasn't a front-runner. It actually sorta faded away, as happens to movies it seems that don't open in November-December. Academy voters have short memory spans.

But then that Affleck "snub" happened, and incredibly it gave fire back to ARGO's campaign.

Who knows? If Affleck had been nominated himself from the get-go, ARGO might not have won Best Picture. That would be LINCOLN probably.

I give props to George Clooney (won a Best Picture Oscar as a producer of ARGO) for not giving an Oscar speech. Incredibly believe it or not, his 8th Oscar nomination and 6th different category to be nominated for. He previously was nominated for Actor, Supporting Actor*, Original Screenplay, Adapted Screenplay, and Director.

*=Which he won his first Oscar for. Can't remember the movie's title off the top of my head.