First off, probably because no one used them since they weren't deemed canon. Second off, I don't see a single adventure class, ship much less a civilian ship, in the Ships of the Imperium that have hardening on the M-Drive, sensors, and weapons. Third, while, yes, it might only last one round, but I'm pretty sure the Ion Cannon can fire every round and a good shot allows it to last D3 rounds. Fourth, so what if the Comp/5 or /10 has /fib. What good is a computer if your power plant is offline?It interrupts the power for a turn. It might kill all the power, but usually results in a choice between maneuvering and firing. And if the free trader pays for EM hardening on the M-drive, sensors and weapons, the ion cannon is useless during combat. /Fib is cheap for a Comp/5 or /10.
The 2Dx10 is consistent with the previous progression, and no one has been screaming about the barbettes being unbalanced for all these years since the initial printing.