The show was cancelled due to plummeting ratings:
Although launching to large audiences of 12.47 million viewers in the U.S., the show's ratings rapidly declined to ratings about one-third that figure.
Meanwhile, V managed to premiere with lower ratings, but they lost less of their audience over time:
On Tuesday, ABC Entertainment President Steve McPherson explained the difficult decision this way, “It was a ‘Sophie’s Choice.’ In the end, FlashForward didn’t engage audiences like we hoped. For V, I get more anecdotal calls and emails than any other show on the schedule. A huge part of rebuilding the network is about taking chances. Some shows just don’t work out.”
Given that all of this happened 3 years ago, and there's been no news of a resurrection of the show, it's safe to assume it's not coming back.