Let's take a look at the actual numbers, here.
Take the Castrum, for an example of what the "assault PWS" can be.
8 Medium and 36 Light Sub-Capital Cannons. 11,200 tons, for a total potential damage output of 132 Capital-scale damage.
A comparable battery of NAC/20s would have about 30% more range, it's true.
But to have the same damage output it would have to mass 50% more - 17,500 tons for 7 NAC/20.
NAC would build up 420 heat, compared to 672 for subcaps
And it would cost 35,000,000 c-bills (and cost 168,000 per combat round of firing, just in ammunition), compared to 18,120,000 and 324,000 per volley's worth of ammo.
So, there are clearly some real tradeoffs to think about here.
Capitals cost twice as much to build, but half as much to operate in combat.
They have a noticeable range advantage, though not as much as over conventional weaponry.
They require fewer crew to operate.
Even after adding in the mass required for additional heat sinks and crew accommodations, the capital weapons pay a significant mass penalty.
Even at 100,000 tons total mass, the Castrum does not have that much extra room to play around with. It cannot afford that mass penalty.
More than that, the subcapitals are more versatile - more accurate when firing on anything other than another warship, for example, and with more of them, more capable of bracket fire and engaging numerous smaller targets.
They can also provide more precise, pinpoint ortillery fire, rather than the massive craters than NAC/20 fire would produce.
And, most of all, given the in-game explanation for why it won't work, they don't require a minimum SI to represent the massive structural bracing required to make them a stable firing platform.
