The current Imperial Navy doctrine of armored prows and torpedoes is a direct result of its degeneration in technology. For every step forward (which is usually a rediscovery of older tech anyway) there is usually 2 steps backward in another area. In the Imperium, older usually means better, not outdated. That is why the older ship designs, such as the ones that defected to Chaos, are faster and tend to have heavier weapon broadsides. Prime examples of this are the Murder class cruiser, predecessor to the Lunar class as the mainstay of Battlefleet Obscuras between M33-M37, the Carnage class, and the speedy Slaughter class cruiser. The dominance of the armored prow and torpedoes paradigm is because it is easier (i.e. lower tech) to slap on slabs of armor and put in torpedo tubes than it is to make and maintain long range firing lance weapons such as those fielded on the Murder class.
Grand cruisers are larger than battlecruisers, which in contrast are basically cruisers with dorsal weapons added. The Imperium still makes battleships and battlecruisers, but grand cruisers are relegated to reserve fleets or mothballed. It is explicitly said the grand cruiser fell out of favor when the Imperium could no longer build sufficiently powerful engines:
The grand cruiser was a precursor to the battlecruisers which are now more prevalent in most Imperial fleets. Almost rivalling battleships in its ability to withstand damage and lay down a barrage of fire, the grand cruiser fell out of favour when the means for constructing sufficiently powerful engines was lost (more recently built engines were incapable of attaining a useful combat speed).
p. 119, BFG rulebook
