Because according to professional ballerina Sarah Lane,
"[Producers] were trying to create this façade that she had become a ballerina in a year and a half."
Lane was Portman's "dancing double" in the movie, and the special effects team used face replacement to make it look as though Portman was doing all the dancing. Turns out the only thing she was dancing around was the issue of how hard she worked on the movie."
Lane reveals in an interview that Fox Searchlight prohibited her from talking to the press until after the Oscars.
Not to say that Natalie didn't still wow audiences with her acting, and I assume everyone knows Hollywood is 90% smoke and mirrors (just look at any recent magazine cover with Britney Spears on it). But perpetrating a rumor for an Oscar win is low.
Despite the fact that she wasn't credited on IMDB for being a double and Natalie Portman left her out of her acceptance speech, Lane keeps a positive attitude.
“It was a great experience to see the whole process of making a movie."
Next thing you know, we're going to find out Sarah is the one who's really pregnant.