You are correct the Gunfire and Mine/Torpedo Damage tables do not provide for a bulkhead hit on BA-BD ships. Note that on the Mine/Torpedo Damage table, B* is in the column AC - AV • B*, so a bulkhead hit on an old pre-dreadnought is exceptionally likely. For rationale, I suspect Lonnie would state something like in general, battleships were big enough and tough enough (protected and subdivided) to such an extent that the damage model didn't warrant a bulkhead result, given all the applicable factors and constraints (e.g. having another row in the Critical Hit table with a bulkhead result might have been nice, but there wasn't room).
As for the results from Taranto, I doesn't seem to me to be an issue. None of the three BBs hit (Caio Duilio, Conte di Cavour and Littorio) were sunk/destroyed such that they would not have been repairable. The fact that Conte di Cavour did not return to service before Italy was knocked out of the war is more attributable to Italy's lack of repair capacity and resources. Just my $0.02...
Dave