we can't see the close edge of the fire, presumably it butts up against the embankment that Tala is watching from. So they would need to go out and around, giving Obi Wan time to escape. However, Vader could have just put out the flames again. But this was his first clue that Obi-Wan was working with allies, so Vader might have decided to let them escape for the bigger picture, in the hopes of putting down more of the Rebellion. That was his plan in A New Hope, after all.
As I understand, the Force isn't unlimited. Force users do get fatigued. It could be that he just didn't have the power to put the flames out.?