It's not from Rogue One, but Episode IV gave a somewhat different explanation--one more about the thinking and attitude of the Empire in general:

Dodonna:

> The battle station is heavily shielded and carries a firepower greater than half the star fleet. It's defenses are designed around a direct large-scale assault. A small one-man fighter should be able to penetrate the outer defense.

Gold Leader:

> Pardon me for asking, sir, but what good are snub fighters going to be against that?

Dodonna:

> Well, the Empire doesn't consider a small one-man fighter to be any threat, or they'd have a tighter defense. An analysis of the plans provided by Princess Leia has demonstrated a weakness in the battle station.

As with most good writing, this does (at least) double duty, so to speak. It not only tells us how this defect was allowed to happen, but also tells us more about the basic character of the Empire in general--rather than caring about an individual and how he or she can attack the station, they think only in terms of large machines and who they can attack the station.

In short, the Empire is basically like a person with a basic character defect--lack of empathy or caring for people. That defect is not only what allows the attack to succeed, but also a large part of what makes the Empire so evil, thereby justifying carrying out the attack in the first place.