There's no excuse not to have the SD 500s
Yes there is one: It would require the introduction of a completeley new damage model that would differentiate between damage by blast and damage by fragmentation, as well as targets reacting differently to those categories.
Currently all our bombs are kind of generically representing the very type of bombs the SC class is belonging to: General Purpose bombs, relying on relatively high amounnts of explosive filling. And currently our ordnance damage model is likewewise generically relying on blast effects for damage determination. If HTC could have given us the SD 500, but without such a change it would simply be a 500kg bomb with the blast effect of an 250kg bomb or even less, due to it's radically reduced amount of explosive filling.
I'm all for adding a more complex system and thus different types of bombs which would be chosen by the player according to his targets: Armor piercing bombs for attacking the cruiser or even a future battleship, general purpose bombs for attacking town and industry structures, fragmentation and cluster bombs for the attack on 'soft' targets.
But until then I don't see much point in adding a 'weak' SD 500 bomb.