Because the longer a launch goes the easier it is. Basically there are critical phases of flight and there's the actual continuous operation of the rocket all the time. Things like clearing the tower, max q, stage separation, engine re-lighting are all insanely complex operations, but once all that's done and all you need to do is burn the engines for longer it's pretty easy to just burn more rocket fuel on a flight that has been working the whole time. its something that is much less risky to the mission going on. Things can go wrong, but the chance is much higher during one of those complex things.