Bombardment cannons aren't the same as macrocannons though. Bombardment cannons fire magma bombs which is why they hit on 4+ and crit on 4+. Last time I checked, IN escorts and Cruisers weren't sporting those stats. I'd also wager that as a assault fleet, the marines embarked on a crusade would be working with IN ships and therefore have access to IN supply lines if not their own supply lines. Surly the marines who, by your estimation, are more than capable of building whole fleets of warships without oversight would have little trouble with the far less complex and resource intensive systems of torpedo.
The whys and werefors of that magic to hit and crit bonus are a bit murkier now, since other weapons stated to fire magma bombs are heaped in with macrocannons, including meltacannons in 'starship' size and quite a few plasma and laser weapons. 'Weapon battery' and 'macrocannon' are pretty broad categories at this point, encompassing everything from the 'standard' macrocannon to such esoteric entertainment as disruptor weapons, including entire spectrums of crap that have their own special rules outside BFG. As are lances, really, if you get down to it.
Personally, I just file it away with the reason a Lunar's stygies pattern macrocannons don't treat the target's armor as a point lower in BFG.
And, as I said: Dominion fleets would have no problem producing torps. Crusade fleets, on the other hand, would find them exceedingly difficult to produce on a regular basis. And, remember, other then regular maintenance, a ship does not have to be rebuilt in it's entirety (usually) after a battle. Given the limited number of munitions that a given ship can carry, they would have to replenish fairly often. It's not a matter of capability, it's a matter of available resources. Transporting torps, or even components, as cargo is hideously risky, to the point of being a hazard to the ships around them, if you're transporting sufficient amounts to keep a Crusade fleet stocked.
How risky? Orks think it's a
good idea to produce torps on board ship, there's that much entertainment potential.