there's no guaranteeing that ONE deflector would be on the right side of the planet to fire the beam in the right direction at the right time.
Now that would depend on time window issues we don't know much about. An incoming asteroid might be at deflection range for several weeks, providing a daily window of intercept for an arbitrarily located single deflector.
Of course, this is another area where the episode drops the ball. How fast did that asteroid move? Many things about it actually suggest relativistic speeds, which supports the idea that the Amerinds were deliberately placed in harm's way, and the harm furthermore was quite "synthetic" in nature.