Though it was heavily defended by blaster weapons, being in the corridor reduced the total number of weapons on the Deathstar that could be trained on the Rebel ships in the corridor. The higher up from the Deathstar the Rebel craft were, the higher the amount of weapons that could be aimed at them from the surface of the Deathstar. So, as deadly as it was, being in the corridor put them below the horizon of most of the weapons on the Deathstar making it safer than not being in the corridor. In addition, the corridor was much easier to locate on the huge Deathstar than the target within the corridor. Following the corridor to the shaft was the surest way to the target.