They don't. They recognize that viewers tend to not watch episodes consistently in December, due to the holidays, and that is unacceptable for a show with a strong story-arc, such as Flash Forward. So there is no logic in presenting the show until at least January.
The Winter Olympic Games will be held February 12-28, 2010. This is event television in the extreme, and there will be especial interest this time because the games are being held on the continent. It is even more optimal, for the broadcaster of the games, because the games are in the Pacific time zone, which means that many premiere medal events can be carried live all over the continent. Again, there is no logic in presenting a show with a strong story-arc, like Flash Forward, when so many of its viewers will be distracted by the spectacle of the Olympic Games.
Swiping the show on and off air, off in December, on in January, off in February, on in March, is also incompatible with the concept of a show with a strong story-arc.
Essentially, all these inescapable realities means that the very best approach to presenting Flash Forward involves putting it on hiatus until the show can be broadcast for a reasonably extended period of time, and that means pushing the next episode out until after the Olympic Games.
Which is exactly what ABC is doing.
A lot of people complain that this is what killed similar shows in the past, like Jericho. However, note that those people never seem to have any proof that the alternative would have been better for the show. There is no reason to believe it would have been.
It is very possible that the nature of today's television viewer is incompatible with shows with strong story-arc.