Actually have a working theory on that...
Porkins, if you disregard the BS EU that he tinkers iwth his ship and it crashed, and if you go with the film's context which IMPLIES he's caught in crossfire from the surface, is the key... Another instance I feel implies my theory's the "reason" is Luke's "getting singed" on his strafe run.
The theory I have working is that the closer you get to the surface, the more intensely tight the crossfire produced by the surface guns is, and the easier it is for the Empire to shoot you down with their gunnery teams.
This applies somewhat to Earth Naval/Air combat from the 20th Cent... Low "Deck" level attacks often were met with more accurate fire, etc... One could assume SW gun turrets are maybe more accurate as well anyway.
The attack run on the trench then requires a sharp dive into the trench, eliminating the majority of the surface guns being able to draw fire on you... The key is the thought that the gunnery crews on the surface ARE accurate enough to shoot fighters down (as the film implies that they do with Porkins, and almost burn Luke too), but that the fighters are maintaining a certain "ceiling" while drawing the Imperial fighters away as best they can, and waiting for run after run to take place.
Some EU helps support it too, namely the flight sims... The closer you get to any capital ship, the more intense and accurate the fire is from its gun turrets. They can really paste you good, and firing warheads is a trick all its own where you have to fly in close and "dumbfire" most of the time to get an accurate hit, as the warheads are relatively easy to shoot down when they're flying at a locked target from a distance.
The exhaust port attack would've been somewhat of a "dumbfire" shot (for Luke it was completely so) with the more simplistic targeting computer helping the pilot manually aim, rather than the EU "lock" system the HUD uses, but which can throw the warhead's aim off... Hard to explain really, but if you've played the game it actually makes sense and jives with the films even though the targeting screen isn't an available feature in the games at all (save for the Rogue Squad games).
In that sense, the attack on the DS1 makes complete sense, but you just have to buy into the DS Gunners being accurate at close range, which I do tend to agree with. I hate that BS EU story that says Porkins screwed his controls up and THAT is why he's offed. Give them gunners some credit dammit!