Characters have a "dice roll" that they need to make in order to hit someone (at least, to avoid "misses", since even after this roll you can still be parried, dodged, etc.).

As you gain in rank, it's easier and easier to make this roll because you have a lower target that you need to achieve.

An opponent's AC makes it harder for you to hit them, the lower their AC is.

What this means is that at lower ranks, a low AC is very good against fighting types because unless they've geared for a lot of hitroll (and few do), they'll just miss with all their melee attacks. But at higher ranks, they have such a low target that there's not a lot AC can do.

There's also a warrior legacy that warriors can pick at level 44 or 48 that makes AC important because lower AC can give them damage reduction.