chapotraphouse

It's all rule of cool. But presumably the exhaust port has heavier defences. In real life flying low in a plane or helicopter makes it harder to hit with air defences because you can't see it for long enough to work out when to fire in front of it or hit it with anything that can track it. Plus radar and other stuff find it harder to identify. So presumably they get close to the death star, swoop into the trench for cover and then try and hit the port with their missiles. If they flew head on either turrets or missile batteries would have got them.