The network only didn't have enough time to air all the episodes because the Fall season is entering.
Despite taking low number of audience each week, "Defying Gravity" will still escape from being cut. According to Futon Critic, the show's publicist, Nicole Marostica, issued a statement that ABC is not giving up on the show and is still figuring out a time slot to air the last 5 episodes of season 1. Marostica didn't give any indication though on when roughly the show will return on screen.
The speculation that the show is canceled after only eight episodes was based on the fact that a preview for next week was not given at the end of the September 13 episode. Plus, the show has been absorbing only 2.5 million viewers in average since the third episode. The only reason that ABC puts the show in vacuum apparently is because it started the summer season late and didn't have the time to air all the 13 episodes.
"Brothers & Sisters" retains its September 27 premiere at 10/9c slot. Meanwhile, for this coming Sunday, September 20, ABC will present feature movie "King Kong" because it doesn't want to battle an original with CBS' live broadcast of 61st Annual Primetime Emmy Awards.