The bomber is designed to take down capital ships. In order for the bomber to accomplish that task, it needs a Supernova. Thats it, there's nothing else to it.
To expand more upon that point: In order for a bomber to fulfill its purpose, its weapon system must be able to down the shields of a Battleship, and perform significant damage to its hull. If bombers were unable to penetrate capital ship shielding, they would be utterly useless.
Then why not have a more powerful torpedo than the nova ?
HOW DOES IT MAKE SENSE for a FIGHTER (bombers are the same size so count in this) weapon to be as powerful as the MOST powerful battleship weapon ?
It doesnt.
Period.
So you want to kill a BS, boo hoo.
get in a GROUP or ROLEPLAY thefact that you SHOULD NOT be able to in a small group take one out (unless the pilot sucks)