The IN does tend to think a little backwards. Their mindset seems alot like the actual gameplay really, theyre stuck somewhere between where we were in ww1 and ww2. Thats why theres not really any true "carriers" in the fleet, no one wants to risk moving away from what they know works.
By 'we' I'm assuming you mean England. Everyone else had a pretty good idea by, oh, 1930. (Though, in the US case, in fighting in the Navy muddled the issue, Bakka's fluff being a not too subtle jab at the US from that period, right down to the disastrous battle where carriers make it out but the big gun ships go down in flames [Circe])
In fact, in fluff Bakka has been very badly mangled numerous times due to a lack of carriers including The Fang (Space Wolves), Circe (nids) and Bakka itself in both 980.M41 (Red Corsairs assault the Navy fortress to considerable effect until Huron is nearly killed by a Grey Knight strike team teleporting onto the bridge of his battlebarge [sound familiar?]) and 999.M41 (Dark Eldar destroy the IN anchorage at Bakka).
And that's the baffling part is that they do know carriers work, and work quite well. (Remember that the 'current' IN fleet is relatively new, with Lunars only coming into service in M37, though the Mars is somewhat older). The 'Big Gun Lobby' would not only have been on Bucharis' side during the Plague of Unbelief (and thus killed by any number of loyalist organizations or as part of the general uprising against all his officers), but due to a serious plot hole, would probably not have existed at all. Remember that at the time, they would still have been using the 'old' IN fleet aka the Chaos Fleet.
One of the mainstays of which is the Styx, which given the time it takes to complete a battleship, would still have been very much in service at the time of the 'Young School's ride to power.
Because the Gothic does perform on its own?
According to fluff it performs poorly during the Gothic War.
With Defiant we've buffed a dozen different ways either for BFG:R or for FAQ 2010.
It continues to suck.
It's time to address the Elephant in the room and try giving it 4 lbs like every other game system it appears in does.