Simple - 2016.
If Obama wins, Hillary can run in 2016 and have a good chance of winning. But if Romney wins, he will likely hold the office until 2020. And if Romney does fix things, a Republican would likely carry the office into 2024. Hillary, if still alive, would be too old. And the Clintons do not want to wait 8 or 12 more years - they already feel they had the last four years pulled out from under them.